Been a while since I have read this thread. Sorry you are still having problems.
Way back on the first page I saw a problem. Perhaps may help someone else:
One or the other of the Maracyns will lower the O2 in the water. When you are treating with these products be sure to increase the aeration in the tank to help with oxygenation. This can be done with an air bubbler, a power head that makes some serious ripples across the surface, or changing the water level so the return form the filter causes more surface movement.
Next, a question:
When you did the most recent total tank cleaning, did you refill with all new water? Here is my concern: If you recently filled the tank with tap water (TDS in the very low 100s) and a week later the TDS was close to or over 400 (Tests seem to have ranged from mid 300s to mid 400s) then SOMETHING has been added to the water to read that high, that much change in TDS. (Meter may read different and the same water could produce some odd readings of plus or minus around 10%, but such a big change on a factor of about 4 times as much is a real change, not a problem with the meter)
I see one person suggested it was the medicine. Might be. Many medicines are a certain % active ingredient, and a certain % inactive ingredients that act as carriers for the medicine, and have other uses. I suppose that there might be enough assorted stuff added with the medical treatment to raise the TDS so much.
It is good to see that small, frequent water changes will lower the TDS. This is the way to go: make such changes in baby steps so the fish can adjust.
If you need to do a larger water change in an aquarium, but the TDS is lower in the new water you can raise the TDS. You could add baking soda (will also raise KH and likely pH) Epsom salt (also raises GH), aquarium plant fertilizers (Especially if you are dealing with a planted tank, potassium fertilizer is a good one) or table salt (not well tolerated by some fish, and not an on-going answer to the problem) Best is to stick tot he smaller water changes until the tap and tank more nearly match.
Without knowing the TDS before you got the meter it is impossible to say if the large water changes that may have happened back then contributed to the problems the fish are having, but it is possible.
Fish regulate the amount of fluid in their cells and their body cavity to maintain a certain level of salts and minerals in their cells. When the water they are living in changes TDS (when the level of dissolved salts and minerals goes down) then more water may try to enter their cells than they are used to getting rid of, and this can cause problems for the fish, and some of the symptoms your fish show might be part of this issue.
Another part of this problem is that fish under stress (disease, parasites, other) have a harder time regulating their body fluids. Their cells become sort of more porous, so excess fluid enters more easily. This is what is behind the idea that aquarium salt eases stress. Salt raises the TDS, so less fluid enters the fishes' cells, and in this way, helps stressed fish. It may be that some of the inactive ingredients in some medicines raise the TDS in the water similar to the way that salt does, and this is part of the treatment.
Next note: Cloudy water after sterilizing the Q-tank.
Nitrifying bacteria do not generally grow so fast as to cloud the water. If you add a bottle of Tetra Safe Start or Dr. Tim's One and Only you are adding enough bacteria that the water may be cloudy for a day or so, but it ought to clear up very quickly, as the bacteria settle out on the filter media and substrate (if any- hospital tanks might not have any substrate).
Usually cloudy water is because of heterotrophic bacteria. These are helpful decomposers, but not directly part of the nitrogen cycle. These bacteria live on organic matter. Some sources include: The oils and cells from your skin as you handled the equipment and decorations; The dead material that may have remained on these surfaces after you sterilized them. The fuzzy patches sure sounds like a massive growth of bacteria or fungi, decomposing something. Cloudy water from this source is not bad for the fish, but it does indicate that there is still material in the tank that might be better off removed.
What I would do now:
If the fish are still showing symptoms that suggest bacterial issues (red streaks under the skin, fin rot, barbles eroding and other issues) I think I would switch antibiotics again. Research exactly which groups of bacteria are killed with the products you have used so far, and figure out what other bacteria type you have not treated for.
http://www.nationalfishpharm.com/fish_d ... teria.html
Here is a link to quite a few bacteria diseases of fish, and the medicine that treats that problem. Usually you would use this reference by starting on the left, and identifying the problems the fish are having. I am suggesting you start on the right and make a list:
You treated with Maracyn II, and this treats Gram negative bacteria.
You treated with Furan 2, and this treats _______.
You treated with Kanamycin, and this treats _______.
What is left over? Is there a group of bacteria that none of these treat? Maybe that is the one causing problems in the tank(s)!
Another good reference is the Merck veterinary manual.
http://www.merckvetmanual.com/mvm/index.jsp
There is not so much specific fish info in the Merck Manual, but there is a lot of info about which drugs treat what sorts of disease organisms. Look up each of the active ingredients you have used so far.
Another thing to consider is that perhaps the problems are not bacteria in origin. Research Mycobacteriosis. The symptoms do not sound like this is the primary problem, but if the fish have this as an underlying problem then treating the fish for some other problem may not work so well.
http://edis.ifas.ufl.edu/VM055
A different sort of approach might help:
1) Maintain 10% daily water changes, with gravel vacs (well, sand vacuuming, it is still called gravel vacuuming) in an effort to slowly lower the TDS. If you can get the TDS in the tank close to the tap water TDS then you can do larger water changes if the need arises.
2) Stop all other medicines, and add an ultra violet sterilizer. This will kill microorganisms drifting in the water. We do not know what is really bothering the fish, or how it is spread. IF there is some infectious organisms that is spread via the water then the UV will highly likely kill it.
Do not use other meds with a UV sterilizer. Many medicines are deactivated with light, and while I have not read it directly, I would assume that the higher energy of a UV light will denature whatever proteins or other complex molecules that make up the medications.
3) If you want to use a medication in the food with a UV, this would probably be fine. In food, the medicine is not being exposed to the UV light. Perhaps an antibiotic that sounds like a possible treatment from your research is available in a food?
4) Keep up the fishes' health with a good rotation of foods including proteins such as fish and shellfish and vegetables. If you make your own food, you could incorporate medicines in that. Be very careful not to introduce more potential disease or parasite problems with contaminated food sources such as live aquatic worms or foods that may come from waters where other fish live, such as: do not harvest Daphnia or mosquito larvae from your local pond or stream. Certainly do not feed live feeder fish.
One more idea, weird, but here goes:
Set up a bucket or a tank with all new equipment, or totally sterilized, scrubbed equipment and start a fishless cycle in there. Grow a really good population of nitrifying bacteria on the filter media. Then, when you are ready to sterilize the hospital tank (or even the main tank) you will have an uncontaminated source of nitrifying bacteria, (No fish diseases, because there are no fish in the fishless cycle) without having to buy more bacteria in a bottle. A square sided plastic storage box is a good substitute for a tank, and the filter media can just be tossed in, loose. A small fountain pump is all that is needed to circulate the water. Feed the growing bacteria with ammonia per the fishless cycle instructions. Do not contaminate this with anything from your fish tanks, or even use equipment or tools in common. If something happens and you want to sterilize a tank with fish you could net the fish, dip them in some concentrated medicated bath and put them in this storage box as a temporary tank. (Do a big water once the fishless cycle is done before adding the fish to this box. The fishless cycle results in very high nitrates)