Actually cat scratches are dirty enough to cause an infection all by themselves. Cat's have bacteria hanging out on their claws because they scratch around in their litter boxes and walk on floors. Their nails are shaped just perfectly to imbed these bacteria deep within your skin when they scratch you. It is important to wash every scratch really well and cleanse it with an antibacterial soap. Did you clean your scratches immediately after getting them yesterday? I bet the cat was the culprit and not so much the fish tank. In order to help draw out the infection, I'd suggest soaking your hand in warm salty water for 20 minutes twice a day. If the infection is really bad, I'd suggest going to your doctor so you can get some antibiotics.
Good luck!
z.