I have had these two rbp in this 20 gallon tank for about 6 months and they have been doing great. I religiously change the water every week and check all the ph and temperature of the water. I plan on getting a 55 or 60 gallon tank soon. How long could i survive with those two rpbs in that tank. Also in that new tank i plan on gettin i want to get a 6 or 7 piranha. What species of piranha could i fit 7 or 8 in a 55 to 60 gallon tank?