In comparing cost I am seeing it's probably going to be cheaper to divide my tank with frosted glass. I have enough sponge filters and heater from the existing tank and the glass is free. I'd have to buy more aquarium silicone. I want to move the water from 5 gallon tanks the fish may gain a gallon. I also have cycled 2 gallons but I am thinking with two gallons and gaining 4 would actually start the cycle over. If I take out my fish and do the change quickly is it likely enough BB will survive not to break the cycle?