I have a 20 gallon freshwater aquarium since september of '20. I have 2 angel fish, 1 red tailed black shark, 2 guppies & 1 algae eater. I also have 3 live plants & assorted artificial plants. Anyway my ph sits around 6.6, ammonia & nitrite are 0ppm but nitrates are 80 ppm or slightly higher. I...