See the other post about bulkheads and using larger powerheads instead of massive iwaki type pumps.
My feeling about these soft plastic semi-rigid containers is that they were not meant to be drilled. And silicone doesn't stick to the plastic!
An alternative is to make a siphon from one to the other using large PVC pipe.
There is probably someone out there that has made these tubs work, so I'll shut up now and let them talk!