where exactly is the placement of the tupperware in relation to your tank's waterline? I made my own canister filters back then using the lock-n-lock containers (get them for about $4 at walmart).
At first the canisters were near the top of the tank waterline and there wasn't much flow...the canisters would push water out faster than it can suck, so it caused the tupperware to contract. After i lowered the placement of the tupperware the issue disappeared.
fyi, it's a pain to service the tupperware. The silicone would loosen each time the tubings move, and eventually you would need to reseal it. In the end i got a xp3 and life was great.