Cloudy Tank, Dead Fish

Tragedy Strikes. Noticed my tank going cloudy about a week ago. Tested water and all seemed well. Did a water change, somewhat cleared it up. Woke up this morning, couldn't see 12" through the water, much more cloudy.

Also this morning, my pictus catfish, who was usually out of sight and cruising around the bottom of the tank was acting unusual. He was swimming around mid to top of tank. He also went to bottom couple of times and went almost down on his side, with mouth open. Got back from classes today and checked the water params (API liquid test kit) and found ammonia 0 nitrite 0 and nitrate 40. Then did a 50-70% water change. Water is still not clear, but better than it was in the morning.In doing so, found the pictus cat dead. Toast. I realize now I should have taken a picture, but he is gone. I did notice his eyes looked cloudy, that's about all I could tell, possibly a white mucus on him, but not sure about that one.

I've been religious with weekly water changes. Try my best not to overfeed, if anything sometimes the plecos dont finish an algae wafer. Only thing I could think of is that I got a new HOB filter, to run on this tank in preparation for setting up my 40b in the enxt few weeks. I put a aquaclear filter sponge over the intake and didn't rinse it off as well as putting a new decoration in the tank without rinsing it off. I thought that was why the tank was cloudy and assumed it would clear up. But with a dead fish I don't see why cloudy water would be the reasoning, and the cloudy eyes make me thing something else is going on.

Dealt with the ICH several weeks ago, mostly saw the pictus catfish with the ICH. treated with heat and salt, went away after about a week, continued treatment for a week. WIth water changes I've been tracking how much salt I've been taking out and after about 3 weeks total I'm down to about .5tsp/gal, down from about 2tsp/gallon.

Any thoughts?
Thanks....

I don't know much, but if my water parameters are still normal wouldn't that not be a stalled cycle? and the only 'meds' i used was salt. one pleco has been going to surface more often, what i assume if for air, hasn't been 'sucking' onto surfaces as much as usual. eyes don't look out of norm. Going to be browsing fish diseases on the forum for a while now.

I've got gravel substrate. I haven't changed it since starting up the tank (20L). Now I'm getting worried that I did something wrong...but I'll be honest. When I do my weekly water changes, I take the water out via graval vac - as in I gravel vac about half the tank really well and that removes about 3-5 gallons out of the tank. Then refill the water in the tank. Then a week later, I gravel vac the rest of the tank taking out 3-5 gallons. I hope that's not incorrect. Am I strictly supposed to take my water change from the water column and not not through the substrate?

I haven't changed my filter media - well when i added my new filter - same as my old filter, i switched the two filter cartridges between the two, but they weren't out of the water for more than 5 seconds. What does all of that mean then? What should I do about this?
Edit....After some research of 'bacteria bloom' yeah that sounds about dead on to me. over feeding. Now what to do.
Thanks

I guess what i should say is 20gL? Sorry for being unclear. The tank is a 20 gallon Long. I posted to the freshwater newbie form, I bought a new tank because I know I'm overstocked or will be very overstocked in future. Glabe mentioned possible concerns of so many bottom feeders in my new 40 gallon breeder still.

The 20 is as follows:
1 Redtail Shark ~2.5-3 in
1 ABNP ~2.5 in
1 Rubber Lip Pleco ~3in
1 Angelfish ~2 in body now, maybe 4-6 inches tall with fins
-2x Aqueon Quiet Flow 30 (One was the just recently added - with a sponge over the intake tube)
-1x AquaTech 5-15

Doing an aqadvisor check, with 'juvenile sizes' I thought I would be okay for a while until I get the 40breeder set up - waiting on a check and a stand. I think I'll be giving up on HOB's and going to big canister.

I understand after looking into bacteria blooms that with the pictus catfish I was probably over stocked, over feeding, and having pleco poop machines, and with some possible stressed out fish. I've been trying my best to make moves on my new 40 gallon. I guess I should just wait out the bloom and stick with feeding less for time being? Maybe larger water changes than my 15-20% weekly?

So increase water changes and easy on the feeding. Yeah even being as inexperienced as I am (!N00B!) I'd agree. Upping the water changes. I feel like you two (bettafishmommy and mesto) are probably very correct....more clean water has got to be good. Thank you all for your help so far. I really appreciate it. Just want to do right by these fish. I admit I wouldn't have some of them if I had done my homework in the past like I should have. But...I like to think I may learn from my mistakes. Thanks again!
-Also put another air pump up for the tank to try keep oxygen levels up. The whole bacteria thing taking all the oxygen from my fish (those lil Bast***s!) IS NOT COOL! So yeah. Felt terrible seeing my pictus cat roaming around with mouth upon, what I assume was gasping for air. Tragic. And not shakespeares tragic comedy, just a tragedy.