Is rainwater safer than tapwater?

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Neale Monks advises a PFK reader on the use of rainwater in the aquarium.

Q) I have read so much about people locally having tank problems, and losing fish even though their water tests are OK. Some seem to be putting the problem source down to something in their tapwater.

Is rainwater safe to use in tropical aquariums — it must contain less additives than tapwater — and if so would you still require to use a conditioner to treat rainwater prior to adding it to the tank?

ALAN VIA EMAIL

A) NEALE MONKS REPLIES: Rainwater can be safe, but it does absorb airborne pollution, and that could cause problems for your fish. Most rainwater is fairly clean nowadays, but if you lived near a factory, power station or busy road, you might want to think twice before using it.

Secondly, rainwater will pick up chemicals from the roof, gutters and water butt, so these need to be as chemically inert as possible. Slate and clay tile roofs are generally safe, but avoid using water from roofs treated with tar, asphalt or other weatherproofing chemicals.

Keep gutters and downpipes clean, and make a habit of routinely emptying and rinsing out the water butt every few months. You'd be surprised how much debris ends up there, rotting away and releasing chemicals such as ammonia and tannic acid.

Do you need to use a water conditioner? Probably not, though if you can detect ammonia in the water, adding water conditioner would be useful — as well as a good sign it's time to rinse out the water butt!

Filtering rainwater through a chemical filter such as carbon or Polyfilter would be more useful because these adsorb chemicals of the sorts likely to be in airborne pollution. Simply stuff a cheap internal canister filter with one or other medium and pop it in a bucket of rainwater. Leave it running for half an hour or so, which should be ample time for the filter to have turned over the water in bucket a few times.

Most aquarists using rainwater use it in addition to tap water. Mixing rainwater 50/50 with tap that has a hardness of 20ËšdH results in water with a hardness of 10ËšdH, which is ideal for a very wide range of community fish. But if you wanted to use rainwater by itself you need to add either Discus Buffer (for soft, acidic water conditions) or Rift Valley cichlid salt mix (for hard, alkaline water conditions). These would provide the appropriate pH and hardness levels your fish need to stay healthy.

With all this said, few aquarists only use rainwater. Although it might be difficult to believe with the weather we've had at late, even in England there may be times where it doesn't rain for weeks, and storing large amounts of rainwater safely is difficult. Those aquarists who really do need to avoid tap water tend to use RO water instead. Although expensive to buy and maintain, RO filters produce very safe, very pure water that can then be turned into soft water or hard water using the same sorts of buffers mentioned above.

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