Hi Robyn

I have an 88Ltr tank which I usually fill with bottled water because my tap water is high in nitrate. This is quite expensive so I recently got a water-butt so I can collect rainwater to use instead. I've tested the rainwater collected. It contains no nitrate or nitrite, is pH6.4 and contains 0.5ppm of Ammonia. Is this safe to use as it is? Is there anything else I need to test it for? I thought the pH was a little high and wondered if it would be ok to add a pH adjuster or would adding aquarium salt be a better idea given its other properties. I also wondered if rainwater with an acidic pH was indicative of pollution but I do live in a rural area so assumed this would be unlikely. Anyhow the soil around here is acidic too. I wasn't sure if it was ok to use the water given it contains a little bit of ammonia. The water in my fish tank contains no ammonia and I wondered if adding the rainwater with some in may disrupt it. I'm not sure why the rainwater has ammonia in it. Is it because it had been standing for a few days? I thought it might be ok to use if I add it gradually as the biofilter in my tank is pretty healthy. I'd really wlecome your thoughts though.
Many thanks
Ros

Does the rain water come from the roof or elsewhere? If it's from a new roof, you may get some bad chemicals coming off the roofing. If it's an old roof, that is less likely. Either way, rain is pretty pure but in some areas of course, it may be acid rain (pH below 5, sometimes even 3 or so, like a can of soda!). With a pH of 6.4, it seems pretty normal for rain. The thing with rain is that it isn't buffered at all. The pH reading will not be stable and will fluctuate. The alkalinity and hardness are almost nothing. If you're mixing the rain water with other water or using it for animals that like water like that, then it's fine. Otherwise, you should add some saltwater mix, water hardness increaser (sold for cichlids, etc.), and/or baking soda, etc. to get to the pH, hardness, and alkalinity that the animals you are keeping desire the most. The ammonia reading may be a false positive, coming from the roof, or present in the rain. You can run the water through some zeolite to remove the ammonia first or just put that in the filter or add a liquid additive to neutralize the ammonia. If your tank is mature, the good bacteria should be able to process the ammonia in short order as well. Be sure to aerate the rain water so the oxygen levels go up as it's often low in oxygen (hence pond fish often gasp after a rain). How big is your tank? In most cases, you probably don't want to change more than 20% of the water to rain water the first few times. Once they're used to it, it can be more like 40% if you're able to match the same amonts of additives (salts, etc.) each time to keep the water chemistry pretty consistent.

I'm not sure either why the ammonia reading was so high unless it was a false positive (more likely in unbuffered water). Oh, the container also may leach certain chemicals depending on what it is. If the rain water sits long enough that life forms start to grow, then that too certainly can change the water chemistry.

Good luck!