A 1/4 inch hole will be plenty for a siphon break. Your filters are not holding their own. They are barely started. In a couple weeks your filters will start to keep up with the ammonia, and nitrite will spike off the charts. Then in another three weeks or so the bacteria that work on nitrite will start to catch up with the nitrite. Meanwhile you should be doing large water changes daily to keep both levels sub lethal. Your fish will be fairly stressed during this time period. Take you salt levels up quite a bit to help counteract the nitrite poisoning. I would not expect the filters themselves to handle the waste from much more than 100 pounds of fish but with the additional help of greenwater, you might be able to handle more. If you are raising 300 pounds of fish your system is going to need to process 6+ pounds of feed per day. That is not going to be an easy feat for your drum filters. So adding more would be a good idea, but you might want to alter the design, and make a couple into trickle filters so the nitrifying bacteria is exposed to more O2. This will be more efficient. Or better yet, use the outflow from your existing drum filters to spin some RBC's. I kid you not, you are in for daily water changes for a month. If you had done a fishless cycle, you would have conserved quite a bit of water, not stressed your fish, and would have sped up the cycle. If I were you, I would put your existing Tilapia in the freezer, do a fishless cycle, and order new fish in a couple weeks. But what do I know?
