I am so sorry to hear of all the troubles you've had. It's pretty frustrating when things keep going wrong over and over again.
It's certainly not edema, because that would show on both sides of the abdomen at the very least, if not the entire body. I'd have to look up the Iridovirus.. not familiar with that one.
As for the nuisance brown 'algae', those are really diatoms, and normally it goes away once it uses up the supply of silicates in the tank. It's quite usual for it to show up in a new tank for awhile. But if it persists, there's a simple fix. Get a Nerite snail or two and they will eat it all. They think diatoms are like candy. They'll clean it off any and all surfaces, and most other algaes as well. They keep glass clean too.
There are a couple of possibilities for the gourami's lump. It may be something like an abscess. If that is what it is, and it bursts to the outside, you'd see whitish stuff coming out of it. That would be pus and once it drains the fish, if it is otherwise healthy, should be able to heal the wound made by the drainage. But if it bursts inward, the fish will probably die, as the infection will get into the blood and cause sepsis, blood poisoning.
Antibiotics might be helpful if it's an abscess, but I would not know which one to get. Since there isn't any way to know if the organism would be gram positive or negative, you'd need a drug that treats both kinds of bacteria.
There is also a chance it's a tumour, but if so, it's come up very quickly, which wouldn't be a good sign, sadly.
Unfortunately, the truth appears to be that dwarf gouramis are rather fragile fish. They are very beautiful, and I used to keep them. I really, really like them and I wanted to breed them. But I could not keep them alive long enough to do that.
I researched them as much as I could, and most of the information that appeared to be reasonably reliable seemed to indicate that these fish tend not to do very well in the long term. It is not anything you have done wrongly, it's some sort of weakness in the fish. Many fish today are highly inbred, which can lead to any number of genetic weaknesses, which we often can't see on the outside and it might be why these fish are fragile. The lovely colour morphs they come in can only be fixed by inbreeding, which is done deliberately, and that does not help. This is done with all kinds of fish, to get new colours or long fins or some other characteristic that is new or appealing.
Some people do have some success with these fish and they're popular, so they're bred in numbers. I wish I could say I think he will get better. I am really sorry to have to say that. But where there is life, there is always hope.
Keeping the water as clean as possible is one thing you can do. Some extra water changes are never a bad idea. And in general, dwarf gourami like their tanks to be very quiet, with lots of plants in the substrate and floating, for shade. They don't like much water movement. They are generally peaceful and can get on with many fish, but are not happy with fast moving or boisterous fish species, who may stress them.
I'll go see what Iridovirus is.
Edit. Ok.. seems iridovirus, from a brief overview, may be one reason so many DG's die, but the one symptom, an enlarged spleen, is not one I've ever seen in any of the fish I had and lost. But it's been detected in over 20% of the fish from some suppliers of the fish.
It is fatal, unfortunately. There are no treatments for viral diseases other than supportive care. A few other fish have been seen with this disease.. Betta fish, which are related to the gouramis, Angel fish and Rams. If you have Angels or Rams and the DG has this disease, there's a possibility they might get it. But I hope you don't have these, they'd be terrible tank mates for a timid DG.
What other fish do you have ?
You've clearly had some rotten luck with equipment, but that's really not the norm. Wish you had had someone to advise and help you with the issues that cropped up sooner, before the frustration factor got so high.
It's sadly true that the best way to defend against illnesses, like ich or anything else, to is have a quarantine tank, where you keep any new fish for at least six weeks to make sure they are not bringing your display tank some sort of grief. So many new fish keepers either don't have the space or are not aware of how important a QT tank can be.
I sincerely wish you the best of luck.. and I really hope this fish will defy the odds and get well.
