So the tetra is fine now. Apparently I misdiagnosed it. There are no more spots and nothing appeared on the other fish. I'm going to do another water test tonight and see how everything is going. The pleco is a common hypo. All o the fish seem to be doing well and everything is going good. You guys are a huge help.

Gravel vacuums are probably the easiest thing to use that you will ever buy for your fish tank.

As for filter cartridges you don't have to change them nearly as often as the instructions say you do. So long as the cartridge isn't deteriorating or falling apart you should leave it. When it is finally time to change it, if you can fit the replacement cartridge in behind the old one that would be great. If not, you can probably get away with replacing it as long as you skip the gravel cleaning for a little while. That way you'll still have a bunch of beneficial bacteria growing somewhere.