Fish on bottom of the tank and aro itchy

All my fish are sitting on the bottom of the tank. It's a overstocked for reasons not relating to this, bit it will be changing soon. Anyways I have been doing 30% water changes once a week or so. The tank has been running for about 3 weeks with a seeded fx6. I just added a bunch of lava rock to the sump, but the problem started before that. Ph is right around 7, ammonia 0, and I haven't checked the other parameters because I am missing those test kits. The only fish swimming around well are 2 of the Florida gars and the oscar. Everything else is just kind of sitting at the bottom. Temp is 82 and I thought maybe the dissolved oxygen content was low, so I added an air stone right in front of the return, but after 5 hours there has been no change. Any thoughts on this? There is no inch present on any of the fish and no other ailments that I can see.

I am on top of water changes. It's just been hectic with setting up that tank and trying to keep track of when I do each water change. I do one at least once a week, but not exactly on the same days. I've got 5 different tanks going and I do water changes differently on each of them.