How do I improve this aged water chemistry?

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Neale Monks advises on a reader's problem with the water chemistry in an old tank.

Q: I’m helping to look after a really old tank for a relative while he’s in hospital, and the fish in it aren’t looking so great. The tank uses an old undergravel filter that is probably older than I am, and when I tested his water with my test kits, the pH was far too low (it looked close to 5.0 on my colour chart) and the nitrate was so high that the water in the test tube turned a colour that wasn’t even on the chart! I’ve carried out a week of water changes at 25% a day, which has slowly brought the pH back up to 6.8, but the nitrate is still above the highest level that my kit can read. Is there a way that I can bring this back down any faster? I can’t really use plants as he has this big gravel in the tank, and I can’t add a nitrate removing pouch into the filter as there’s nowhere to put it. Please could you advise?

PAUL STANTON, VIA EMAIL

A: Neale responds: It’s quite normal for tanks to become more acidic as time passes. Indeed, one of the many reasons that frequent water changes are recommended is to offset this acidification by removing the acidic chemicals periodically. Among other sources of acidity are nitrate and phosphate, which accumulate because of biological decay, and the leaching out of organic acids (such as tannins) from decaying plant material, particularly wood. Now, if you have hard tapwater, the impact that these have will be negligible, but in soft water areas it doesn’t take much for the water to become much more acidic than we’d normally want in even a Southeast Asian or Amazonian biotope tank. 

With that said, so long as the pH change was slow, many soft fish can handle a pH as low as 5 without much trouble. But fish that need hard water conditions, as with most livebearers, can become sickly in low pH conditions, developing problems like finrot and fungal infections.

So, what you do next will depend a bit on your local water chemistry and the fish you’re keeping. If you have ‘liquid rock’ water that is hard and alkaline, then a series of modest water changes (I’d suggest 10% every couple of days), should slowly bring the pH back to normal. If you live in a soft water area, then the use of a commercial pH buffer is probably the easiest approach here.

I wouldn’t add directly to the tank though, but instead add enough to each bucket of water for that specific amount, so that you’re only nudging the pH slowly up to where you want it, a pH of 7 being ideal for a mixed community tank. What you don’t want to do is make a big change in pH. Even going from a ‘bad’ pH to a ‘good’ pH will stress fish if it’s done too quickly. 

Much better to make the changes slowly, so your fish can adapt, ideally over a week or so.

Now, the one thing I’d not go crazy about changing is the filter. Undergravel filters work well, even if they’re a bit limiting in terms of plants, rockwork, and suchlike. Plus, if you’re diddling around with water chemistry, uprooting the filter is just another thing to stress the fish. I’d give the gravel a modest clean, syphoning up any detritus, though I’d not stir more than the top 1cm, but otherwise leave it alone. 

A series of water changes should bring the nitrate levels down, and if the tank has been neglected for a while, the lack of water changes is probably why the nitrate is so high. If you wanted to use a nitrate removing medium, the best bet would be to pop one inside an air-powered box filter, hook it up to an air pump, and let it do its thing. 

These box filters are cheap, don’t take up much space, and are easy to remove and clean when required.