Yes there is about a 1/4 inch gap. Considering they are 41qt tubs and for the larger fms I went with a larger gap for better ventilation without drilling holes. I've had the first one running for about 8 months now without any problems, but you could easily shave 1/8in off your 4 blocks and have an 1/8 inch gap. Just personal preference. If you plan on putting small males in there for breeding might as well be safe than sorry!









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