Oxygen Levels

[davecpayne]

Hi, looking for any input on oxygen levels in fish tanks and tap water.

As background, I recently returned to fishkeeping after several years without a tank for various reasons - I previous had a largeish tank (180L from memory) tank which I ran successfully for many years, first as a community aquarium then (after a short break) with African Cichlids. So, I'm not a complete beginner, but also well aware that there's plenty I've forgotten, and even more than I never knew.

So - my problem - in the first half of last year, I set up a new, small (50L) tank, as a freshwater community tank. Fishless cycled with an external canister filter rated for 100L, white sand substrate, a couple of real plants and 3 or 4 fake ones. Ammonia and nitrites were down to 0, nitrates were low, and I gradually introduced a fairly high number of fish - 14 neon tetra, 8 guppies, 2 otos and 2 honey gouramis. I realise that was probably over-stocked, but with the high level of filtration and weekly 25% water changes, the ammonia and nitrites stayed at 0, with nitrates getting up to 30/40ppm pre-water change.

Everything seemed to be going fine, with the slight exception that I had a lot of algae growth, the white sand was almost competely covered in algae, some of which I'd vacuum out with each water change. I don't think I was over-feeding (feeding every 2-3 days), and mostly put the algae down to having under-estimated the amount of sunlight in the spot where the tank was. It's in a very bright, but North-facing (in the UK) room - no direct sunlight, but lots of indirect light. The fish didn't seem to mind it and all seemed to be healthy and happy.

Then at the start of November, over the course of about a week, all the fish except for the tetras died. I first noticed that the guppies were gasping at the surface, so did a big 50%+ water change, and repeated that over several days, as well as directing the outflow of the filter (which is a long bar, more than half the width of the tank) upwards to agitate the surface of the water a lot. Nothing helped though, and everything except the tetras died. The tetras seemed completely unmoved by the whole thing.

So - pretty sure that was down to oxygen levels, but I'm confused about why.

Nothing had changed, except all the algae on the sand died off shortly before. Complete guess, but given the time of year, I assume that was down to the days getting shorter, and therefore there being less daylight in the room. Could it be that the algae photosynthesising had been injecting enough oxygen into the water to keep the fish healthy?

Since the die-off, I've bought an oxygen test kit (Tetra), and it consistently shows 2mg/l both in the tank and in my tap water. I figure that means water changes won't be materially increasing the oxygen level? Is 2mg/l an unusually low level of oxygen in tap water (in the UK?) I also vaguely wondered whether something to do with the change of season means our tap water is coming from a different source, so maybe I'm starting with less oxygen in the first place?

My only real thoughts about resolving things are:
- add an air stone - not opposed to this, but my understanding is all they really do is agitate the surface, and given the outflow from my filter is already agitating well over half the surface, I'm not sure that it will make a huge difference
- more plants - definitely an option (I love the look of heavily planted tanks, but always struggled with plants in the past)

That all ended up quite a lot longer than I intended, thank you for reading to the end! Open to any and all suggestions, corrections and contradictions for any foolish assumptions I've made.

 

[GaryE]

Monitoring oxygen was a good idea, but I think your problem was the uptake of oxygen by the fish. You were severely overstocked, and a die off was coming. It always happens.

In a 50 litre tank, I would consider the 8 neons alone to be a dangerous population level, calling for weekly 40% water changes, with no other fish. If you get healthy neons, they are tough little creatures. You were lucky with the ones you bought, because disease issues from the farms have made many of them delicate. When I say they're tough, a lot of newer hobbyists will question if I've learned anything in 6 decades with fish, but well raised ones or wild caught ones are solid little fish.

Test kits are great for establishing the cycle, and great for those who enjoy doing the tests. But once the tank is cycled, they don't tell you a lot. The nitrogen cycle is crucial, but it's just one of many processes going on underwater. Oxygen distribution is another.

Generally, if you push the limits on stocking, something is going to give. A minor gill parasite that would normally be just there suddenly becomes a killer, a bacterium that was held at bay runs wild, a virus under wraps explodes out. We tend to focus on the water these days, where older aquarists always looked just to the fish with parasites and diseases. The middle ground is where we'll end up, and it will be a good thing.

So, I think your tank has achieved its balance, which you can maintain with water changes and basic maintenance, You were on track, but with a massive overpopulation problem. That's worked out, in an unpleasant way. You're now okay with stocking, but can't add.

The cure is a larger tank.

 

[davecpayne]

Thanks very much for your detailed reply.

I was initially concerned about the level of stocking, and introduced them gradually for exactly that reason. I don't remember ever having had problems with oxygen levels with previous tanks (even allowing for the passage of time and my generally poor memory), but I suppose that was more down to the tank size and therefore surface area.

I'm not doubting your advice about stocking at all, totally get your point that something was going to give, and will stick with the level I'm at - maybe I'll try and hone my gardening skills to add a bit of interest (no offence to the neons!). That said, I'm surprised at how suddenly the oxygen became a problem - it really was practically overnight that they went from seemingly totally fine to all gasping, and then dead within a few days.

I'd really like to understand the chemistry/biology of that, although I appreciate there's probably too little information here to be able to explain it with any confidence. I'm also curious to see what happens with the algae as the days get longer, and whether it has any noticeable impact on the oxygen levels.

 

[TwoTankAmin]

Did you test even though is is suggested above not to need to do so once a tank is cycled. What you describe comports with nitrite poisoning. Gasping at the surface is the typical indication that is is likely the problem. Without repeating all the science to explain how this all works it is enough to know these facts. There can be other causes of the surface gasping but since nitrite is the mors common cause it pays to test for this ASAP when you see fish behaving this way.

Nitrite prevents blood from carrying oxygen. This is why no matter how well oxtgenated the water is, the fish are suffocating.

Nitrite enters the fish via the gills.

Chloride, blocks nitrite from entering the fish.

Salt is sodium chloride.

So a small amount of salt protects dish from nitrite. Water changes are not the best way to deal with nitrite Here is why:

Once inside a fish nitrite takes a day or two for it to work its way out of the fish. But as long as there is still nitrite in the water and no chloride has been added, the nitrite keeps entering the fish. So water changes are not the solution for nitrite the way they are for an ammonia issue.

This site has very precise and clear directions for how to determine how much salt to add for any given level of nitrite. This information can be found here https://www.fishforums.net/threads/rescuing-a-fish-in-cycle-gone-wild-part-il.433778/ Note- the anoung of salt need to counteract nitrite is pretty small and almost never an issue compared to the harm being done by the nitrite.

One last observation re the above. Because it takes time for nitrite to work its way out of a fish it is possible that a nitrite spike occurred and natural forces caused the nitrite oxidizing bacteria to multiply and handle it, there can still be nitrite left in a fish even though it is gone from the water. The only way of which I am aware fo counteracting nitrite alrady inside a fish is by adding Methylene Blue to the water. This is pretty messy and usually the use of salt is enough to fix things.

 

[MaloK]

A healthy range for dissolved oxygen in a freshwater aquarium is generally considered to be between 6.5 and 8.0 milligrams per liter (mg/L).

I asked a couple months ago about oxygen tester and was surprised that nearly nobody use them.

A sponge filter with a tall riser and good air pump is going to create a lot more gases exchanges than just an air stone alone. in the air lift tube.

There's some algae strains also that have a noticeable impact on oxygen levels during night time and was surprised that O2 was fluctuating that much.

And I also bought an oxygen tester and now am certain that my tank runs at near max oxygen all the time.

 

[davecpayne]

@TwoTankAmin thank you for another great reply, that's really interesting. I do continue to test, typically about every other water change, for no real reason other than that I'm a nerd and like spreadsheets

Looking back at last November, I can see that I tested about a week before the die-off and about a week after. I'm not sure why not at the time itself – or maybe I did but forgot to record it? Either way, nitrites were definitely at 0ppm just before and just after – in fact other than the expected spike during the fishless cycle, I've never seen anything above 0 for nitrites.

Do you think that means nitrite poisoning is unlikely? Or might there have been a spike (possibly caused by the algae die-off) which the fish took up, and then I removed from the water with the big water changes, but (as you say) not from the fish? Kicking myself for not doing a nitrite test at the time now!

 

[davecpayne]

@MaloK - thank you too, especially for the ideal numbers. I think that's about the same that I read in the test kit instructions, so glad to have it confirmed that 2mg/L is low.

I don't suppose you know what your tap water's level is? Would be interesting to know if your water starts from a higher concentration than mine, or if the difference is your kit.

I hadn't seen sponge filters, so shall go and have a good read up now, thank you!

 

[MaloK]

@MaloK - thank you too, especially for the ideal numbers. I think that's about the same that I read in the test kit instructions, so glad to have it confirmed that 2mg/L is low.

I don't suppose you know what your tap water's level is? Would be interesting to know if your water starts from a higher concentration than mine, or if the difference is your kit.

I hadn't seen sponge filters, so shall go and have a good read up now, thank you!

The levels out of my tap is 4 mg/L 02 and it also contain a little C02, when the CO2 vents it bring also the oxygen down. I aerate the water in my holding tank 5 minutes per day. When I take that water the O2 is around 7 mg/L

And the aquarium with 2 filters holds between 6.5 and 7.

Also other factor is temperature and salinity that can have a great influence on dissolved O2. In addition that some water chemistry can have a lower oxygen solubility to add to the equation.

Take a sample of your source water in a bucket and aerate it solidly with an air stone for a couple hours and test the O2 level for fun.

 

[davecpayne]

Take a sample of your source water in a bucket and aerate it solidly with an air stone for a couple hours and test the O2 level for fun.

Ooh, I like this idea. I shall definitely experiment, thank you.

 

[MaloK]

Ooh, I like this idea. I shall definitely experiment, thank you.

When you try it, also test PH, It's going to give a good idea on how the chemistry will result in your tank.

 

[TwoTankAmin]

If the issue is nitrite and the levels have gone down to 0 in the tank, then the fish should recover inside a day or two. If it is something else then the fish will continue to struggle to breath. Ammonia at sub-lethal levels can damage gills.

 

[GaryE]

Are the neons in distress?

In a tank as crowded as that, I can see nitrite poisoning as a problem. In a more lightly stocked tank, your water change routine would have made that poisoning unlikely. When I dissed test kits, it was because once the cycle is established and a regular water change routine is too, you don't get the build up, unless you seriously overstock or do water changes in a haphazard way.

You also lost some anabantoids, the honey gouramis. This raises questions, because they are surface air breathers, an adaptation to very low oxygen levels in their native habitat. If any fish is going to survive oxygen level issues, it's them.

It was such a selective die off. That's strange. Otos are delicate fish that need clean, oxygenated water and a lot of biofilm to feed on. Store bought fancy guppies tend to be wrecks - like neons a once indestructible fish that is now weak and vulnerable due to farming issues and chronic diseases. So I can see why the more fragile fish would die. But the neons surviving is interesting. Odd.