Established tank cloudly out of the blue, why?

Question
I saw someone ask a similar question to this but I have a few differences and was hoping someone could help me.
Yesterday I noticed my fishtank was just slightly cloudy. So I did a 25% water change. The water remained cloudy even after the water change. I was hoping it would clear up overnight, but when I came out this morning it was almost completely white. I freaked out and opened the lid. My two 4 year old big Raphael Catfish were floating at the top. I ran and grabbed a cup to scoop them out, but thankfully when I nudged them, they were still alive.
When I did the waterchange yesterday, I also tested the water. PH, Ammonia, Nitrates, and Nitrites were all normal or low. Normally my Nitrate level is a little high so that confused me.

Anyway, after seeing my Raphaels in trouble, I did a 60% water change. The water is still quite a bit cloudy. I also was devastated to discover I lost a 2 year old Bleeding Heart Tetra, my 2 two year old Kuli Loaches and my huge 2 year old Clown Loach. They still were quite bright colored so I assumed they died pretty recently. I actually kept them in seperate cups for a while hoping they were in shock and not actually dead, but after a half hour they hadn't moved. :(

My water levels are still normal, I have no idea what is wrong. I did replace my Eclipse filter this morning as well. My BioWheel is fine and I have never replaced or cleaned it. I haven't added anything odd to the tank and no poison could have gotten in as far as I can tell. Things seem to have been as normal as thy've always been.

Here are the details of my tank:

Size of tank and amount of fish: I have a 38 gallon tank and I have now two 4 inch Raphael Catfish, two small 1 1/2 inch Clown Plecos, one 4 inch regular pleco, two 2 inch Black skirted Tetras, two 1 inch Dwarf Golden Gouramis, and three 1 inch Hatchet Fish. I also had two 3 inch kuli Loaches, one fat 3 inch Clown loach and one 2 inch Bleeding heart teatra till they died this morning.

How long has your tank been set up? About 6 years

How often and how much water do you change at a time? I usually do a 25% water change once a month.

What type filter system do you have running? I have the Biowheel and the Eclipse 3 system,

Has the filter cycled yet? and do you have filter media/ material like a bio-wheel for the beneficial bacteria to live in so that the nitrogen cycle or "biological cycle" is not destroyed with water changes & routine maintenance? The tank cycled many years ago. The Biowheel has never been changed or cleaned.

Could you test your water and give me the ppm readings for nitrates, nitrites, ammonia, Gh, & Kh along with the Ph and temperature of the water?

The levels are at normal meaning:
Nitrates: 10
Nitrites: .25
Ammonia: .25
PH: 7
Temp: 79

From the tap, do you have hard or soft water? I'm in Colorado, I think our water is pretty normal but on the hard side.

What do you use to condition the tanks water? I normally use AmQuel Plus,  but I was out of it so I used Ammo-Lock yesterday and Stresscoat today. This was the first time I've ever used AmmoLock but I've used Stresscoat a bunch in the past.

Do you add anything else to the water? Normally no. The past week I had been using a little Melafix because one of my Kuli Loaches had a bit of a growth on his head. I've used Melafix many times in the past with no problems whatsoever.

What kind of substrate are you using and is it planted with live plants? I have plain purple fishtank gravel thats been in there for 6 years. I have 2 live plants that have been in there for at least a year.

What kind of food and how often and how much do you feed? I do vary my fishes food. I just finished off using TetraMin and got some new food last week. I'm using OSI Freshwater Flake, Hikari Sinking Wafers and Boyds Vita Diet. I alternate between the OSI and Boyds everyday and don't feed that much. I use the sinking tablets for my bottom feeders 3 times a week.

I am completely devastated. I have no idea what to do. We've always had a helthy happy tank so I have no idea what could have happened. :( Any help would be appreciated, and quickly too since my fish are not in the clear yet.
Thanks!!

Answer
Thanks for so much helpful information. Your ammonia is a bit high along with nitrates as ideal is 0 for both but they are not bad as you know. Food you are using does not usually ever cause cloudiness in water if fish are overfed, but I know that is not an issue. Recently I have had numerous people contacting me regarding cloudiness in water and have recently used Ammolock I am beginning to wonder if something in the Ammolock is disrupting something in the water chemistry causing the problemm, however that is only a guess and nothing more. Age of the fish may be a small factor but again, with the excellent maintenance and attention to your tank I do not think that is related either. You mentioned your nitrate is much lower than normal - sudden changes in water chemistry can cause problems like sudden death because the fish have been used to and more tolerant of higher levels. I have had that problem when I first began keeping fish. Your kuhli loach and clown loach would be the most sensitive to any changes in the water chemistry.
I wish I could pin point the issue but I really can't and won't attempt to pretend I can. Monitor your water chemistry for any sudden changes, check your heater to make sure that is functioning properly, you may also want to add an additional air source with this odd occurance in your tank to ensure they have plenty of oxygen. I suggested checking the heater because I once had one malfunction unknown to me which caused the temp to rise and cause cloudiness, some thermometers are more reliable than other but you have virtually the best for your fish so I assume you have a good thermometer too.
Again, I wish I could have been of more assistance. Best of luck and happy fish keeping.