Nitrate Issues

[Olycius]

I currently have a 125G tank that has a Fluval FX6 filter. I have been feeding my fish(50 small-medium community fish) the same amount for about a year. It is heavily planted, and the has plenty of light on a timer that runs for 10 hours a day. Nitrite and ammonia are 0, and hardness and PH 7.5 are normal.

Recently I noticed that my nitrates spiked up a lot from about 10ppm to 60-100ppm which is bad. I quickly did a water change of 20%, and it really did not help. The next day I did another 30% and it helped a little, but the numbers are still above comfortable. I do not want to clean the filter as I just changed a lot of water and do not want to throw the balance off for the other parameters as I want the beneficial bacteria to have a chance to stay at a healthy level.

Is there something else that I could do to help with this? The plants in the tank are doing just fine and are growing well and so far only one fish has shown signs of being sick. I'm thinking of not feeding them for a few days to see if that helps. All of the fish are eating normally and are showing no sign of illness besides the one that has been moved to a sick tank.

 

[seangee]

Have you tested your tap water for nitrates?

 

[Olycius]

Have you tested your tap water for nitrates?

Nitrates are minimal from my tap. I have 4 other tanks that had their weekly water change yesterday and their nitrates are 10ppm.

 

[fluttermoth]

Larger water changes are the quickest and easiest way to reduce nitrate in a tank, as long as your tap water is low in nitrate of course!

Changes of only 20 or 30% aren't big enough to change nitrate levels; you really want to be doing 50, 60, or even 75%. As long as the new water is warmed and dechlorinated, large water changes will not affect your filter bacteria, although do check the pH of your tank and tap water first, as tanks that have high nitrate can sometimes have very low pH, and that will need lots of smaller water changes to rectify, rather than a few large ones.

 

[Olycius]

Larger water changes are the quickest and easiest way to reduce nitrate in a tank, as long as your tap water is low in nitrate of course!

Changes of only 20 or 30% aren't big enough to change nitrate levels; you really want to be doing 50, 60, or even 75%. As long as the new water is warmed and dechlorinated, large water changes will not affect your filter bacteria, although do check the pH of your tank and tap water first, as tanks that have high nitrate can sometimes have very low pH, and that will need lots of smaller water changes to rectify, rather than a few large ones.

Since I did water changes on Saturday and Sunday, do you think it would be alright to continue with a 50% change today?

I did 25 gallons the first day, and 35 gallons yesterday. So would it be a good idea to do 60 gallons?

 

[fluttermoth]

Yes; just as an example, all my tanks (except the shrimps) get 50% as a baseline; if I have a heavily stocked tank, that gets pushed up to either 50% twice weekly, or one of 75%, depending on how the nitrate looks (I feel I should point out that there are other reasons for doing water changes, apart from reducing nitrate, it's just that nitrate is a good indicator for things we can't or don't test for).

As long as the tank hasn't been neglected for a very long time (leading to what we call 'old tank syndrome', where the water has become very soft, acidic and very high in nitrate), then large water changes are nearly always a good thing.

 

[Olycius]

Yes; just as an example, all my tanks (except the shrimps) get 50% as a baseline; if I have a heavily stocked tank, that gets pushed up to either 50% twice weekly, or one of 75%, depending on how the nitrate looks (I feel I should point out that there are other reasons for doing water changes, apart from reducing nitrate, it's just that nitrate is a good indicator for things we can't or don't test for).

As long as the tank hasn't been neglected for a very long time (leading to what we call 'old tank syndrome', where the water has become very soft, acidic and very high in nitrate), then large water changes are nearly always a good thing.

I'll go ahead and try a 50% then. I shoot for 25% weekly on this tank due to it being so large. I have my good old 5 gallon bucket that I haul from the sink to the tank. Unfortunately, the sinks I have are waterfall sinks so I cannot connect a line directly to the tank as they have no threading. I have had this tank for close to 3 years and this is the first time I have had a spike like this.

 

[seangee]

You won't affect your filter by doing large changes so its fine to do them everyday, subject to what @fluttermoth said about pH.
A couple of questions that may help pin down the cause:
How often and how much water do you usually change?
How often do you test for nitrates?
How often do you clean your filter (and when last)?
Have you added anything to the tank recently (plants, substrate, change in ferts etc)?
Are the nitrates still going up or are they staying fairly stable after each water change?
Has anything died in your tank in the last few months?

Nitrates are the end result of the filter dealing with decaying organic waste (unless introduced unintentionally). The tank doesn't sound overstocked and the FX6 will comfortably cope with a much bigger tank and bioload, and of course with such a large capacity you wouldn't notice a reduced flow if there was a lot of organic waste inside it.

So possible explanations may be (and these are just possibilities, I don't have enough info to know)...

1. Nitrates have steadily been building for some time. This can happen if you have a lot of organic matter inside the filter or in the substrate. The filter will comfortably cope with the additional bioload so there is no spike in ammonia are nitrite so there have been no obvious signs of distress to the fish
2. Something (plant or fish) has recently died in your tank. Again the filter was comfortably able to cope with the ammonia and nitrite spikes but there is nowhere for the nitrates to go.

These aren't the only reasons, I recently had house sitters in when I went on holiday. I left written instructions for feeding which involved counting out pellets and only feeding every other day. Turned out they felt sorry for the fish because they "looked hungry" so gave the "a bit extra" - and of course I had excess nitrates. As your ammonia and nitrites are both zero the filter is clearly established - so you won't do any harm by giving the media a rinse.