Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Storage boxes are usually waterproof, but square/rectangle ones will bow out.
The round storage bins with rope handles work well (don't bow out) but no lid. (Make something up). Smaller containers (like 5 gallon buckets) are fine if it were just a 20 gallon tank, but a 75 holds a lot more water, so you would have quite a few buckets to keep warm. I would look into larger containers.
In any of these solutions you will have the problem of ammonia removal.
If you share the filter among all the containers, share plants (with light!) on all of them, and share the top layer of substrate where the most bacteria live you will be sharing the bio media (bacteria and plants) the best you can. Run the filter on the largest for water circulation, and use other things (air bubbler, power head, smaller filters... whatever you have) to keep the water moving in the other containers.
Largest single container that will hold water:
Stock watering trough. You could fit all the fish in there, run the filter, store the plants, and use the cover from the tank (presumably a glass lid) and light. Might have to rig up some cross pieces to support the glass lid(s).