Phew! Glad to hear things are working out for you. I wish I could have jumped on this thread earlier, but it's finals season and all so I haven't been on the forum as much as I normally am. Anyway...
Those filters are Marineland Bio-wheel filters. Very good filters, in my opinion. The actual Bio-wheels should never be replaced. As far as the filter cartridges - those shouldn't be replaced until they start to physically fall apart. If they get gunked up, you can swish them around in some water you remove during a water change. As for the carbon, definitely keep it out of your tank while you're medicating or it'll suck the medication right out of the water. If you want to run carbon on your tank on a regular basis, take a sharp knife and slice open that blue sponge along the top edge of the cartridge. Then, buy yourself a big jug of carbon from the LFS. This will allow you to replace the carbon however often you'd like without having to replace the entire filter cartridge and thus compromise your bacteria colonies.
I would guess that you lost fish due to both the stress of adding more fish as well as the water being so cold. Also, I didn't see any evidence of ich in any of your pictures. Ich looks exactly like someone poured salt on your fish. Some ich medications can be very harsh on certain types of fish, like your tetras, so that could be causing problems.
I would add those fake plants back to the tank - the fish will be a lot more comfortable if there are plenty of hiding spaces and stuff to break up lines of site in the tank.
Glad things are back on track, though!