Yeah looks like he's got some finrot. Fin rot is black edging to his fins, does it look that way in real life?
Well here's the treatment so that you can start if you like. You'll need Aquarium salt (found at any petstore/walmart or Rock Salt but needs to be PURE NaCl, no additives whatsoever!)
• Take a small cup and put some tank water or conditioned water into it.
• Measure out 1 teaspoon per gallon of salt. (so if it's a 2 gallon tank, measure 2 teaspoons)
• Mix around in the cup until you see absolutely no more crystals (it can burn your fish).
• Add a tablespoon of the dissolved salt into your tank every 15 minutes so your fish can adjust to it.
• Once a day do a 100% water change and re-add water of the same temp that you took out. (Tip: if you run a thermometer under the tap it will give you a rough estimate of the temp, let it run a while so that it can get to a steady temp because Tap's like to vary temps a lot)
• Re-add in the salt you took out. Salt does not evaporate so if you took out all of your water, replace all teaspoons of salt with the above method. (again 1 teaspoon per gallon)
• Continue the salt and water changes for two weeks MAXIMUM or until you see the black edging go away.
Note: Do NOT use AQ salt for more than 14 days, it will cause liver/kidney damage if used long term but works perfectly fine as a treatment for those few days!
But the main thing is you need to keep up on water changes, looks like those tanks might be 1 gallon? That needs to be changed every 3 days with either 50% or 100%. That's why bigger tanks are easier to maintain because you only have to do one water change a week if it's something like a 10 gallon, only 50% once a week whereas smaller tanks need more changes so things like Fin Rot don't happen ^_^
Hope this helps and any questions feel free to ask