Rainwater To Fill The Tank ?

First thoughts would be that there are two problems.  First, ensuring that you have a method of collection that will not contaminate the rain water; Second, rain water is, I think, water of zero hardness. This would make it more akin to RO water than tap water and Third, atmospheric pollution would probably cause you grief too... First thoughts would be that there are three problems...

"Nobody expects the Spanish Inquisition!"

Personally, I would run a mile!

Thank you both for your replies,  Steve I do have a water butt that is filled with rainwater that runs off my shed roof, but I don't think I would want to use it.   I agree that it would be too soft for most of the fish, and would hold lots of organisms that would need too much treating,   but it was a subject that came up in conversation, and I knew I could get a sensible answer on here.    Thank you.  I just thought (wrongly) that there would be more good bacteria in it as it hadn't been chlorinated etc. that might have been useful.

Etienne  you are right spring water from bottles would be far too expensive wouldn't it.    But thanks, it has solved a query anyhow.

Yes. But.

Let me explain; I apologise if you know some of this stuff but some of the others might not. RO water is tap water that is forced through a special type of physical filter; a bit like one of those micropore water-proof jackets. This filter has "holes" that allow water molecules through but not nitrates and phosphates and carbonates and all the other dissolved solids that are normally present in tap water.  The result is almost like pure distilled water. It is quite wasteful; it requires 2-4 litres of tapwater to make 1 litre of RO water,. Not a good idea if you are on a metered supply. You can make the stuff yourself using an RO filter or buy it as required from your lfs.

Now, this stuff is pretty useless outside of a chemistry lab because you've removed all the useful stuff. It would kill your fish in a heartbeat. You have to add back the dissolved salts you have removed, it's called remineralisation. It can be a very effective way of handling problem water because you get to choose which salts you replace and how much of each.

For your nitrates problem, mixing RO water and normal water fifty-fifty would give you water with half the nitrates, it would also have halve the hardness. This wouldn't be a problem for me, I have hard water, but you may not. In fact this is about the only sensible way of softening hardwater.

If you want to try using an RO/water mix, I suggest you take it slowly. A sudden change to a 50:50 mix at one water change might be too much change for the fish, unless it was a really small water change. The way to do it would be either a series of small frequent water changes using the 50:50 mix or your usual water change regime using say 10:90 RO: tap the first time, then increase the proportion of RO at each water change till you get to 50:50. This might be very much on the over cautious side, but I prefer to do things slowly rather than risk harming my fish.

Sorry to mislead you... The 50:50 split thing was an example only. I didn't mean to suggest you leap straight in!  Good catch Sue!

If I were you, I would consider another type of fish. There are, quite literally, hundreds of different corydoras or similar fish, and most of them are not hyper-sensitive to nitrates.

Once you start down the RO route, you are pretty much committed and it will involve a lot of extra legwork and expense for every water change.

I do the RO thing in my 55litre tank. 2:1 Tap:RO It brings the hardness down to 11^o from 16.5^o, but as it's only a small tank I only use 3 litres of RO a week and costs me £1.40 for 10litres, so I'm OK with that. I wouldn't do it for a larger tank, though. It brings the Nitrite in the tap water (25ppm) into the orange on the test kit and the plants bring it down even further.

On RO water - I'm not sure how many aquarium shops offer this, but my local  charges for the containers (which you can keep) but provides RO water free.

When I do a large water change I have to buy containers I don't want to keep, but then they let you return them for in-store credit which I use to get things I was planning on getting from them anyway.

A lot of local tapwater is much too harsh for any fish, so maybe this kind of offer isn't available in softer water areas.  It makes sense for them as they get loyal clients, some of whom would have abandoned fishkeeping, or not started, without that service.

I'm planning on mixing in more tapwater later (currently and all RO tank with additives), but I'll still need 10 - 20 litres a week from them. I get it fortnightly so fewer trips.