How can I tell if my fish has a bacterial infection or a fungal infection?
When I fed my fish this morning, I noticed that one of my brilliant rasboras was quite pale, and his body looked almost like it was covered in a thin layer of slime. A few short stringy strands of slime are hanging off his body. (They’re not very long.) Only one fish is affected, and he was behaving normally – eating, swimming, etc.
I had to be to work before the pet store opened, but I will stop by on my home. I will, of course, ask what they think I ought to treat with. I have Melafix, but I didn’t put any in the tank. If the infection is fungal, I figured it wouldn’t do a whole lot of good, and I’d rather treat the fish properly than guess at it and do more harm.
Anybody have any idea what this infection is, where it came from, and how to treat it?
20 gallon
2 glass catfish
3 brilliant rasboras
1 pineapple swordtail
1 susnet platy

Here’s the site Mudfrog Illegally copied her "answer" from
http://www.ehow.com/how_7030_check-fish-bacteria.html
It’s actually a bit more complex than looking for a few little signs and referencing a few different diseases.
If your fish has a gold or rusty looking shine that’s not normally there it probbaly has Velvet which is a parasite. You can treat velvet with any good parasite or ich medication, I personally prefer Maracide.
Aside from that it’s most likely a bacterial infection. Fungi usually move much slower and give a significant "Fuzzy" look to the fish. An external bacterial infection can be treated well with Maracyn 2 or Maracyn TC among other antibiotic drugs. Since it is external and in the slime coat I would recommend you treat the entire tank and not move the fish to a quarantine tank. Treat as the package recommends and you should see significant improvement within 2-3 days. Don;t stop the treatment then though, treat for a full 5 days to insure the infection is completely killed off.
If I can help further please feel free to email me.
MM
See if the fish appears sluggish, hides, is not eating, has a swollen body (dropsy), red spots, red streaks or is bleeding on its body. These are all signs of an internal bacterial infection. One of the most common symptoms is dropsy, where the body bulges at the sides and stomach and the scales may even be forced to stick out. Bulging eyes, or "pop eye," is a common symptom of an internal bacterial infection.
Watch to see if the fish sinks to the bottom of the tank or floats at the very top of the water, but is not dead. This could be a swim bladder disorder commonly caused by bacteria. Head-standing, laying flat, floating upside-down or sideways, or poorly coordinated swimming are also signs.
Look for black patches on the fins and body along with body swelling. This is a bacterial infection called myxobacteriosis.
Watch for small black specks on the fish’s body. Black spot, or diplopstomiasis, often occurs after the addition of new fish to the tank.
Suspect fin rot if the fins are whitish and ragged. Fungal and bacterial infections may also cause cottony white patches on the edges of the fins. Fungal fin rot often follows damage from an aggressive tankmate. Bacterial fin rot is similar, but not cottony. It’s very contagious and can also be caused by poor water quality.
Look for a white or gray material covering just the eyes. This is an eye fungus.
Good Luck