How should I adjust this water?

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One reader would like some advice on how is best to adjust the water in her tank. Neale Monks gives his advice...

Q) I am setting up a 30cm nano cube and I’d like to keep a Betta in it with a small shoal of either Ember tetras or Chilli rasboras and some Cherry shrimp.

The tapwater has a pH of 8.0 and a KH of 18; the GH is 9. The tank has been running a week and I know I’m a way off adding fish yet, but wanted to use this as an opportunity to get the levels right. I know the pH and KH is high for Betta, so do I need to reduce this and if so, how?

The information I’m finding online gives very mixed advice and the implication is that most fish will adapt to the water rather than me adapting the water to the fish, but I’m not sure I’m very comfortable with that. Peat isn’t really an option and neither is lots of driftwood or catappa leaves, which only leaves a chemical solution, but how stable are these treatments?

EMMA BRATLEY, VIA EMAIL

A) NEALE MONKS SAYS: The tetras and rasboras come from naturally soft and acidic environments, whereas commercially farmed shrimps and Betta are a quite a bit more adaptable. In terms of an optimal balance for all concerned, something around 10°dH, pH7.0 is about right. Your high pH level reflects your high carbonate hardness, so yes, you will want to lower these.

I’d suggest simply mixing 50% tapwater with 50% RO or rainwater should probably do the trick, at least as far as general and carbonate hardness go, and while the pH will probably come down to about 7.5, you could help it along and lower it a spot using a commercially available pH buffer.

Let me stress that by themselves, pH buffers are not a solution to hard water, and should never be used without lowering the mineral levels in your water by adding RO or rainwater. Since your tank is so small, you may have more than enough rainwater for this project, but if collecting and storing clean rainwater isn’t possible, some retailers sell RO water by the gallon.

The problem with chemical pH buffers is that while they lower KH, the resulting salts produced by the acid-base neutralisation reaction results in an increase in dissolved minerals. You end up with hard but acidic water, which really doesn’t match anything most fish are exposed to in the wild. On top of that, this approach tends to be unpredictable and unstable. There’s just no real advantage to approaching water chemistry this way.

You are quite right to be skeptical of the idea that fish adapt to water chemistry values outside their optimal range. While some fish evidently can do to some degree, others are much less adaptable. Chilli rasbora are true blackwater fish and unlikely to do well in hard water for long, and the Ember tetras, while not quite so finicky, will certainly look their best in soft, acidic conditions. Betta, at least the farmed ones, will adapt reasonably well to anything between 2-20°dH, pH 6-8, although they do appreciate water towards the more neutral, not too hard end of the range.

Cherry shrimps are similarly adaptable, but again, neutral, not too hard water is best for them. But Betta can, and do, eat small Cherry shrimp.

The shrimp also prefer cooler water than Betta, which are very much hothouse flowers. Similarly, where the shrimps appreciate currents and well-oxygenated water, Betta are fish from still water habitats that struggle to swim against any sort of current at all. Personally, I don’t think they are particularly compatible tankmates. Indeed, the classic Siamese fighting fish type of Betta with the long fi ns is really best kept alone as it tends to do so poorly in mixed species set-ups.