Hi! I am new to fish tanks however I have the complete basic set up. Filter. Freshwater. Heater. Etc. I have a 20 gallon tank with 1 mono (the other died) 3 glofish, 3 Rainbow Guppies, 1 albino pleco, and 1 shrimp. The tank is about 3 weeks old. The other day I have been noticing one of the...