Hopefully your display tank is open (no lids). I'd be real worried about O2 exchange.
If your main is open, (and this is a general question to the no sump because of evap people).. why do you think you get less evap from the main (with prob a larger surface area, under stronger lighting) than from a sump?[/
The water level stays constant in the main due to replacement volume provided by the return pump. The pump pushes the same amount of water out of the sump all the time so the evap losses in an open main tank are quietly compensated for without use of the 5 gallon bucket. Use a canister filter only in the main tank with no lids and watch the water level drop. Close the main tank with lids and watch the waterline drop. Let the main sit open with no pumping and watch the level drop. The display tank still naturally loses enough water for a topping-off to be done under the above conditions. I switched from cans to the sump for more filter capacity. The main tank held its water level just fine but the sump uncovered the return pump every 3 days. So I covered it as carefully as possible with shrink wrap, tucking it in around the two drain hoses and the return hose. I tucked the wrap over the outside of the sump all around and made it belly into the tank over each section so that evaporated water would hang there and drip back into the system instead of leaving the system as vapor crawling out of the tank under the shrink. Water vapor being what it is I still have losses but it only needs fixing about every 6 days now. Nothings perfect. O2 exchange seems to be fine with lids on main and sump. We still use bubblers in the main tank, the fish are very active and none have blue lips that weren't born with them. Filter Mechanic
Can's are great if you to run media for a short time (or long term) .. carbon, purigen, gfo, whatever.. just take the sponges out or you'll battle nitrates even more.
