Ha ha real funny. But seriously water all down the wall and under your tanks that you can't get to really sucks and we've had 2 earthquakes recently big enough to send water over the sides and thats with the level down
To the top!! So close in fact that sometimes when I close my lid it's barely touching the water I also have two HOBS and they do just fine. I like it because i get more water, HOBs are quiet and no unsightly water lines.
I leave a small gap so the bichirs don't hit the top of their tanks when they surface, but it's only a 3/4" gap, so it's about to the bottom of the plastic rim on the 75 gallon
Well except in my 170G, it's got the overflow box and if I go all the way up the water overflows my sump. Just as well as I've got a bichir and gar and they need to breathe.
all my tanks are atleast 1/2 an inch below the trim*.. we have lots of earthquakes which is my main reason.. however as the tanks and fish have gotten bigger ive found it also helps cut down on injuries.. if theres a 1"-2" gap between the water and the egg crate theres a smaller chance of my fish getting scared up while feeding or when the cat/s spook them..
*with the exception of the 10gallon snail/shrimp/duckweed tank and thats just because i keep the tank from becoming a snowblobe (snowblobe with duckweed)