Hi and welcome to AA.
Just a bit more info needed.
-Firstly, have you cycled your tank before you added fish??
-What way are the fish divided between each tank, how many and which fish are in which tanks??
- How much water are you changing lately???
- Are you adding water conditioner when you refill the tank??
If the lfs
advised you to get 2 Clown Loaches in a 70ltr(18g), I would question any other advice they give you. The problem with SOME lfs
is unfortunately they are more interested in the sale of the fish than the welfare of it. If they die ->you buy more. Extremely poor practice but unfortunately it does happen. The loaches need at the very minimum a 55g and for 2 a 75g. You can get away with it for now, if they are really small but you are going to need rehome them pretty soon.
You really need your own liquid test kit. Again if it's the same lfs
telling you your waters fine, I have my doubts. My guess is there is an issue with your water, high ammonia, nitrites or nitrates. This WILL lead to fish disease and/or death.
Sorry for being the bearer of bad news but now that your here on AA, you will get some proper advice. Whatever your issue is, don't worry, it can be fixed...