Look at your biggest fish. The tank should be at least 4x as long as that fish AND be at least 1.5 x as wide as that fish is long.
If those conditions are met, the fish has enough space to swim reasonably comfortably. When the fish has grown so that those conditions are no longer getting met... you need a bigger tank. In order to avoid buy many tanks its better to use the adult size of the fish for making that calculation. In a 55 gallon tank the limiting factor is its width. At 12 inches wide, you are limited to no fish longer than 8" (This does NOT include eels obviously). Another poster here said it took him 6-7 years to grow his clown loaches to that size.
I am not sure 12 CL at 7-8" can be filtered enough, but 3 of them could.