Water change volume vs frequency

[seangee]

My normal routine for the community tank is a 60% weekly change for the usual reasons. I only do 40% in the nano because that has shrimp. My tap water also has 50ppm nitrates and so far I have been filtering the source.

Recently I have been experimenting with alternative methods of nitrate reduction in my 3 fish homes. The first and easiest of these to crack was the nano. This is by the long way the heaviest planted (its also the newest) and has now reached the stage where it is able to reduce the nitrates from 25ppm to 0 within a week.

Starting today I'm planning to do 2x25% changes because I think it would be better for the fish and shrimp to have a more stable nitrate level as (for nitrates) water changes are the times when I am putting bad stuff in. I also suspect it may be better for the plants to have a more regular supply of nitrates. I guess I would also need to do a periodic (monthly?) large water change (using filtered water) because the smaller changes will be less effective at diluting organic waste, hormones, etc.etc.

Any flaws in my logic?

 

[Colin_T]

sounds good to me

 

[Byron]

I'll begin by saying that in ideal circumstances, by which I mean when nothing "bad" is in the source water and the source water and tank water are basically the same with respect to parameters, larger changes are much more effective than multiple smaller changes over the same period of time. In other words, a 60-70% change once a week is more effective than doing two 30% changes during the week. Water conditions will be more stable with the larger single change. Again, this is only with respect to the water conditions affecting fish; the larger change removes more "crud" than will smaller changes even if there are more of them because with smaller changes you are leaving behind more "crud" than you do with larger changes.

The problem here is the nitrate. You mention "filtering the source," does this mean you are somehow removing some/most/all of the nitrate before using the water?

The reduction of nitrate in the nano is something that will not likely be seen in larger tanks with fish. In the nano, there is next to no ammonia/ammonium being produced, so the plants are forced into using the nitrate. In tanks with fish, this is not so common, as the plants prefer ammonia/ammonium, and only when that is less than sufficient will they turn to nitrate. Plants have to change the nitrate back into ammonium in order to take up the nitrogen, and this is more work so it is something of a last resort. I am of course thinking of low-tech or natural planted tanks here; in high-tech systems with intense light and CO2 diffusion and daily supplementation of nutrients, nitrates are the nitrogen source added and plants are forced into using it as the balance is so high.

I haven't answered your initial question, but I thought it worth mentioning these aspects.

 

[AbbeysDad]

Byron is correct - the single larger weekly partial water change is more effective than half the amount twice a week.
I found it better to do a single weekly WC with filtered water, even if it's 25-50%.

 

[Colin_T]

The problem Seangee has is the 50ppm nitrate in his water supply. By doing a smaller water change twice weekly, the nitrates in the tank won't go up as much when he does a water change. This will be less stressful to the fish and also give the plants a more stabile food supply. The plants will get less nitrate twice a week rather than 1 huge amount each week.

Obviously the best option would be to do big water changes with water that has no nitrates but that is not always possible so we make do with what we have. I personally think his idea is the most practical solution for his situation.

 

[AbbeysDad]

Obviously the best option would be to do big water changes with water that has no nitrates but that is not always possible so we make do with what we have. I personally think his idea is the most practical solution for his situation.

I disagree - he has the Polini (sp) water filter to remove nitrates from his source water. I feel, (like me) that he should continue using filtered water weekly for water changes at 25-50%. Or maybe he does a small mid-week water change with unfiltered water, then a larger weekly water change with filtered water. (Fast growing floating plants can also indirectly significantly reduce tank nitrates!)
High nitrates in source water is becoming more and more common, especially in agricultural areas. Now we all see high nitrates in the aquarium as bad, but don't realize that nitrates in the aquarium "keep bad company" with all sorts of other pollutants we can't/don't test for. So my point is that high nitrates in source water is actually better than high nitrates in aquarium water (as it has less 'baggage'). However, filtered water w/o nitrates is the best path.

Footnote: My position comes from fighting high nitrates in my well water for nearly 20 years for weekly water changes (we drink only bottled, purified water). In my case, I 'invented' a de-nitrate filter using a (now discontinued) converted API Tap Water Filter filled with API Nitra-Zorb. I fill 5g buckets to use for water changes, then a bin and pump to refill tanks. I get nearly 300 gallons of filtered water before I need to recharge with a brine solution.

 

[Colin_T]

point taken. I didn't realise he had access to a denitrating tap water filter. In which case try to use the filter on the tap water and do big water changes as often as you can.

 

[seangee]

Yes I do have a pozzani filter and yes I do understand that 50%+ changes are needed for effective dilution.

I also have 50ppm nitrates in the tap water and will continue to explore alternatives. 50ppm is the EU limit and in a world where cost savings are paramount this will be the norm for a lot of people. Filtering for the tanks is no big deal and I am happy to continue doing so if neccessary.

My third fish home is not quite so simple. That one contains 7000 litres and is at the bottom of my garden. In fact it is at the bottom of the entire neighbourhood. Ignoring the practical and cost implications of a 50% weekly change using filtered water there are years when I am unable to do any water changes for months at a time. When it is wet the water table rises above the level of the pond. So any attempt to drain it means the water pressure will push up the liner and the only place for the fish to go would be my lawn.

Fortunately this is most likely in winter when the fish are not fed and not very active. The fish are all healthy, all over 10 years old and range in size from around 18-24". They do however have plenty of shimmys (spelling?). This in itself is not a cause for concern and I estimate its at least 10 years since I have lost a fish due to any cause. But the shimmys are an indicator of poor water quality and I would rather like to fix it.

 

[Colin_T]

For your aquariums just set up some 200 litre plastic storage containers. Fill them with tapwater, add dechlorinator and put some floating plants in them. Give them light and let them use the nitrates. A week later the nitrates should be gone and you can use the water.

 

[AbbeysDad]

Remember that we'd like tank nitrates as low as possible, certainly <=20ppm.
With good housekeeping, and fast growing floating plants, you might be able to use 50/50 filtered/unfiltered water for water changes. This extends the use life of your filter. (again, nitrates in your tap water are not really the same as nitrates in your tank.) As for the pond, have you tested that water? I'd think that rain would dilute enough to make a difference...and again, PLANTS.