Hard water will not harm your oscars. What kind of water conditioner are you using? Would you also be able to post the ammonia, nitrates, nitrites and PH of your tank here? What temp do you have the tank at? Is there anything that could be contaminating your tank? What are you feeding them and how much?
i use no conditioner i have only a stip that has all the tests on one strip and they all fall in the safe range i keep the tank at 25C, feed pelletes once a day and a small cube of brood worms
25c is ok. Thats 77F. You really need to use a water conditioner. It takes out all the chlorine and chloramines. If not it can take days to get rid of it. It also replaces their slime coat. That may be the problem. Do you have well water? Does your test kit test for all the big ones mentioned before?
Quick question. You say you have kept fish in the past. May I ask what kind?
Yeah get water conditioner, I use Novaqua works great.Gets rid of chloramine and chlorine. From what I know, test strips dont test chloramine, only chlorine. I also use test strips myself and to my knowledge, theres no test strip that has all the tests. The strips with multiple tests on them normally don't include ammonia tests. So, double check your test strip to see if it covers ammonia. If not get an ammonia test strip because this might be a small case of ammonia poisoning. If it is ammonia poisoning, I recommend getting a power sponge filter in your tank (a powerhead that draws water through a porous sponge), its good for biological bacterial growth which is needed if you only have canister filters.