Bulldog Faced plecos work really well on algae and they stay pretty small about 5" to 6". They are also really peaceful dwelling fish. As for the 30 gallon tank for a pleco question, that depends on the type of pleco. The most common pleco is the common pleco. They are a dark brownish with white spot looking things. The common pleco can grow to over 12" long! A 30 gallon tank is defiantly not big enough for a common pleco.