Fungus is not bacterial, just fyi. But you're right, there are fungal spores in every tank, just as there are many types of bacteria in every tank. If there is a wound, or stress, which depresses the immune system, such organisms can take the opportunity presented to cause problems. They're called opportunistic infections because of this.
Wounds often get fungal infections because any cut does break the protective slime coat and allows the organisms access.
As for a quarantine tank, 5 or 10 G is enough for most needs. A honey gourami is small, so does not need a large tank. It's probably easier to maintain good water quality in a 10G, but it can be done in a 5 too, just keep on top of it.
Many people keep a spare sponge filter in a running tank to have handy in case they need to quarantine a fish.. just drop the sponge filter into the QT and you're ready to go.
As for treatment, that's tougher. I have never had to treat fungus, so I've never had to find anything to use for it, and I've no idea what's available in the UK. There are some over the counter antifungal treatments here, so I assume there are in the UK as well. I am not at all certain how good they are, but if it's a good brand name, one of those would be worth trying.
The main thing is to keep the QT water extremely clean, with extra water changes, to reduce the number of organisms that are in the water. Keep the tank bare bottom, so any poop or leftover food can be removed quickly. With any luck the fish will get better. You can also do extra water changes on the main tank to help reduce any fungal organisms in that tank. Extra water changes are not going to harm anything, most fish appreciate them. Just match the temp and be sure to treat for chlorine or whatever is in your tap water.
The small gouramis are shy, so provide some sort of floating cover as well as something the fish can hide under or behind, to reduce stress levels. You can use live plants if the treatment is not going to affect live plants or plastic plants, or pretty much any light plastic item that might float and give some shade.. a yogurt tub lid might work nicely for floating shade. Keep the lighting low in the QT as well.
Did you see any wound on the gourami before the fungus showed up ? The rubbing on objects is called flashing, and it can be caused by external parasites, and parasite damage can be followed by fungal problems. Do you have a picture of the fish with the fungus ?
Rams can be pretty sensitive, and so can the small gouramis. Though they are lovely, they are not that easy to keep, need heavily planted tanks with lots of floating cover plants as well, and are highly susceptible to a number of diseases and infections. They need absolutely pristine water always. They want quiet water conditions, not much water movement, and no fast swimming tank mates or pushy tank mates. Males, and they're nearly always males, females are rarely seen, can also be quite territorial, and may damage other fish too. Though small, they are gouramis, and have that side to their nature.
This is a site with some info on treating various diseases. Have a look, see if any of the meds are available to you. Maracyn is widely available, I wouldn't use Meth blue alone, and anything with meth blue or malachite green in it will stain the silicone and almost anything else it touches too, permanently. So best used in a QT only. Follow the directions carefully, do whatever it tells you to do for the best results.
You can't have carbon, or activated charcoal, in the filter while using any meds. If you use cartridges for the filter, you'll have to remove them, as most have carbon in them or cut them open and dump out the carbon.
You can replace them with filter floss temporarily. Some meds may kill the bacteria in filters too, so be aware of that, it should say on the label if it will harm filter bacteria. Don't let any water from the QT get into the main tank.
Best of luck. The site is here.
Fish Disease and Treatment
