I agree it's not your nitrates. High nitrates cause chronic problems, not sudden acute ones - like death. Dirty filter media, mulm on the substrate, and overfeeding (and overstocking, of course) can all be the cause...it doesn't sound like slack water changes are really your problem.
Dirty filter media? How often/how should I clean my filter? I just replaced one side of the filter with a new cartridge, but I didn't rinse the filter itself out. Do I need to? It's a marineland biowheel 400.
Cutting back the feeding to just once a day, period, would probably be the easiest way to lower your nitrates, or you could break it up into two very small feedings. Fish don't need to eat much at all, they just like to, the same way we doThey don't spend any calories keeping themselves warm, like mammals. They are evolved to go after food whenever it's available, since in the wild food is scarce...so they will eat all they can whenever you give it to them. Best to just keep it to once a day, or twice with very tiny feedings. Frozen food is great to feed, but if you don't sift the "juices" out of frozen bloodworms or frozen brine shrimp, that adds a lot of pollution. I feed whole cubes myself, but I don't feed it often...just about once a week.
Last night I didn't feed anyone, but starting now I'm feeding only once a day
If fish are just disappearing, it's very hard to say what they're dying from. I would just keep doing more water changes than usual and not add anything to the water besides Prime, at this point. Until there are symptoms to treat, medication isn't very helpful.
This morning, I found a dead blue tetra
No one has any signs of illness, swimming, happy and healthy. I don't plan to add any more fish to that tank, only reason I did before was I wanted to up the cories and glofish schools. I'm wondering perhaps it was just the cories themselves, because the 1 peppered cory that made it is larger then the ones that died, and the glofish was the smallest one as well, and same with the blue tetra? Given it's the smallest that are dieing, does that mean something?Could of it been the shark? I got the 55 gallon set up (up til 3:30 am, but it's done!), I moved Anya in there, along with the shark, just in case he's the culprit. And the gold gouramis, they were getting bullied by the blue ones a lot last night (it's after dark I noticed it). And the synos and Roxanne (bn pleco) just because if it is the water.
I know I should of waited to put everyone else in, but Anya has showed no signs of illness, and I was hoping maybe by moving the gold gouramis, shark, synos, and pleco, then that would help and maybe the fish would stop dieing if it was due to incompatibility issues.
So far, so good though. The blue gouramis have seemed to mellow out with the golds gone, they aren't really chasing each other much! Like the female swam by the other one, and nibbled at a leave, next to the other, and NO CHASING!!!
And the kuhli loaches are swimming out and about again! And in my new tank, Anya is much more social and not as skiddish either, I think having other fish helps . The gold gouramis too are being more social. 
