I run an aquarium with four severums, two clown loaches and convict cichlids. At the moment I’m using reverse osmosis water with a buffer. Can I slowly switch from RO to tapwater safely without harming the fish? If so, your advice on how to do this would be great.
R. M. ABBOTT
TROPICAL
NEALE SAYS: The answer to this really depends on the chemistry of your tapwater, and to some extent, its quality, as both of these vary depending on where you live. In Scotland, most homes have soft, slightly acidic water, that’s great for tanks containing fish from the Amazon basin or the rainforests of Southeast Asia (as well as being ideal for the production of Scotch whisky). On the other hand, many parts of England have water with high hardness and an alkaline pH – more comparable to the sorts of conditions experienced by fish from Central America and the Rift Valley lakes, and with a little salt added, is ideal for brackish water fishes too.
A further complication is its quality. While the tapwater in the UK is fine for drinking, in some places the nitrate levels can be quite high, and while 20-40mg/l might not do humans any harm, it’s far too high for many cichlids and some other nitrate-sensitive fish, such as mollies and stingrays. It doesn’t really matter how often you do water changes if the new water has high nitrate levels, and as a result these fish may be disease-prone and difficult to keep. On the other hand, so long as you keep up with your regular water changes, the commonplace tetras, barbs, and catfish don’t seem nearly so sensitive to nitrate levels.
Like a lot of Southeast Asian species, clown loaches prefer soft water, but can handle medium hardness without too much bother. I’d suggest anything between 2-12°H would be fine, alongside a pH of somewhere between 6.5 and 7.
