I am convinced that there's something wrong with my tap water.?

Do not use distilled or spring water. You can use bottled water labeled "drinking water." That's just bottled tap water. However, I do not think your water is the problem. There should be nothing in your municipal water supply that would cause these reactions. It is treated to be free of micro-organisms that could cause illness. Water conditioner should make it perfectly safe to use.

Tell us more about your routine maintenance: what you do, how you do it, and how often. Do you test your water? What are your readings? What size tanks and what are they stocked with?

When we know these things, we may be able to pin-point the likely cause of your problems. My first guess would be improper/inadequate tank maintenance, and/or overstocking. These are just guesses, though. I don't have enough info for a diagnosis or recommendations.

Much too small tanks for those fish, I'm not really surprised that there is problems.

So far the thing that stands out is that they are incorrectly stocked.

Did you cycle these tanks?

If you didn't that may well be part of the problem.

Also you are not changing enough water it should be 25% and the tanks should all be much larger, more than double their current size.

It could be from too much nitrate, since the fish have survived this long you tank is most likely cycled so yet again; nitrate = not large enough water changes & over stocking

i appreciate this question. except you stay in an area the position tap water is taken into consideration "questionable", there is no reason now to not drink tap water. All ingesting bottled water does is make the agencies wealthy. we are led to believe that you won't be able to triumph over bottled water, yet i have considered better than one article coping with this topic, and in each and every case, the author stated that tap water is in basic terms as sturdy because the bottled stuff. attempt buying one in all those britta filters or another such device on your kitchen tap. bypass to a plumbing shop and note in the experience that they can recommend a reliable one. i understand what you mean because I once rented a house on the sea coast in N.C. a lengthy time period in the past and the water tasted undesirable. attempt the clear out, it would value better initially, yet will ward off funds by ability of the years. Salut!!

do you normally do water changes?

you can call your water treatment company and ask if anything has recently changed... but the reason there is chlorine in tap water is to kill stuff like that, I doubt it's the tap water

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I'm confused, you say you do partial water changes twice a month but don't say how much, then you say you do 10% once a week?

25% once a week is the standard... I'm sorry but all your tanks ARE overstocked.

danios need to be kept in groups of 5+ and are super active, need 10gallons+

plecos get huge (assuming it's a common) and are messy, need 55gallons+

goldfish get huge and are SUPER messy, need 20gallons+ for fancies and 55gallons+ for commons... alone it is overstocked but adding more fish means it's super overstocked (not to mention both danios and cory cats should be kept in groups).

strips are known to be inaccurate. I'd suggest liquid tests for ammonia, nitrite and nitrate.

Did you call the city or wherever you get the water from? Do you do water changes at all? Have you tested the water yourself? Is this new? How long have you had fish? Have you tested the water in the tank? Why are you blaming the water? LOTS of questions before we can answer yours.