my fish are turning brown! help

We don't really know what is going on.. it started with serpae tetras, we are pretty sure they got septicemia when the tank was taking a VERY long time for it to cycle. They slowly starting dying one by one.. then we started to notice our dwarf rainbow looked like pieces of poop coming off of his face.. they didnt look like worms though. At the same time we noticed our fish looked like they were turning brown. The pet store told us to get food for internal parasites and as of last night it looked like they were going away some. Well this morning the dwarf rainbow is dead. Pretty much all the fish are brown looking still. The large ornate rainbow we noticed was face down and tail up in the rocks, still breathing but obviously something wrong. The only thing we noticed was that he was getting really brown. It seemed like the area around every scale was brown (like an outline of brown around every scale) and his fins were turning brown especially his face. None of the fish have been acting weird though just turning browner and browner. ONe of the turquoise rainbows looks like he has a piece of lip coming off too as of this morning. The fish store then said to get an antibiotic so we just started using that buy by that time the ornate rainbow is dead and the otto has an eye missing! we don;t know what is going on at all.. it seems like several problems but we can't find anything online either. HELP!!

This is what is in the tank (45gallon already cycled and all levels are perfectly normal):
1 farlowella (algae eater)
2 cory cats
2 turquoise rainbows
1 peacock rainbow (did have two but one died today)
1 blue arcara cichlid
1 german blue ram
3 ottos (one of those is missing an eye now)
2 giant danios
1 ornate rainbow which is now dead
6 serpae tetras which slowly died off one by one in a period of a month to month and a half

p.s. when soaking our very large piece of driftwood in a huge container we used regular iodized salt.. could this be coming out of the driftwood and killing them? or could the driftwood in general be making some sort of mold or fungus form on the fish that could kill them?

well that the thing.. we're not really sure. it was a huge piece that we got in the turtle section at the store. it was just a box with all different sizes of driftwood. its not grapevine or anything like that but it is a pretty dark brown and it does not float whatsoever. we had to drill it down to a piece of slate to make it stay. are there certain types that can kill fish?

they didnt start when we put the driftwood in the water though... it started about two weeks after

yeah the wood didnt have any stickers or anything on it when we bought it just a taped pricetag on it. i looked at the box that they were in and there wasnt anything on that either.

anyone please help

Update as of tonight:

i think because of the antibiotics we put in there and since we had to take the carbon filter out the water is VERY VERY VERY cloudy but now most of our fish have white patches.. it looks almost like their scales came off or something NOT ich white spots cuz we had ich in another tank and it looks nothing at all like it. The fins look kind of raggedy too. Any ideas or guesses or anything would help because we put a lot of money into most of those fish.