Could you be a bit more specific? Api makes a lot of products
If it's stress zyme it's just live bacteria
If it's tap water conditioner I lts not very good, I would suggest getting seachem Prime, and an api master liquid freshwater test kit, the thing is with what you posted there's no real number as you've stated 0-? On many of them, ammonia/ nitrite should always be zero, ph varies but you want a stable pH as most fish (NOT ALL) will acclimate to it, but a pH drop will kill them, as for gh/kh this will not make your fish stay at the bottom gh is what makes your pH not crash unless you have very soft water, yes gh (calcium) is important for the fish as some fish are hard water some fish are soft water, but again it won't make your fish stay at the bottom, nitrate should be between 5-40ppm most say 5-20 but I've had my nitrate as high as 100 without deaths (no this doesn't mean it's okay).
If you're dying water changes every day I would recommend you to not do that, with those fish and that size Tank i would do 2 30% a week unless you get a bad result on ammonia /nitrite then I would do one immidiately .
Is your light extremely bright if so this could be causing them to stay at the bottom.. Do they come up when you have the lights off??
The most important test is not on that strip test kit you have and that is ammonia, and also the strips are very inaccurate as previously mentioned, your pH from that strip looks about a 7.0, nitrate looks about 10-15, nitrite looks to be at zero, how long has this tank been set up?
