I have found a way to create a third tank for breeding...it's probably something most of you already do but...

I used my "cash cows" to keep building money and finally bought everything there was to buy in Supplies. Then I built up a reserve a cash to have on hand. Once I did that, I stopped breeding the "cash cows" but kept them in Tank Two in case I needed to make more money. Now I use the selling tank purely as a breeding place and I can have twice as many fish!