My 240 gallon tank has a built-in overflow with dual Dursos for the drain. I've always had to throttle back my Super Dart to match the drains, but not a huge amount. Also, previously if I blocked the air holes at the top of the pipe it would start to siphon and drain much faster. Lately, however, I noticed my display tank water level was way too high, and the water level in the overflow is at the same level. I had to throttle the return pump back significantly farther to not overflow, but while I'm getting water from the drains, it's not just trickling, I can't get it to drain fast enough to reduce the water level beneath that of the display, and plugging the air vent holes seems to have no impact. Any idea what's going on? Surely if one of them got clogged, the other one could still go into siphon mode....I'm at a loss. Anyone see this before?