Ehhh operator error lol... I bumped one of the lines refilling my res one day and didn't stick around long enough to notice that the return water was now hitting the pump line and being directed to ... elsewhere... not the res...

Luckily I own the place so it's a matter of not being a lazy bum and getting up on my Little Giant (those things are bad ass!!!).

Plus, as I might have stated earlier in the thread, one of the major advantages to the waterfall NFT is the res size can be quite small- even in the 5 gallon pail I ended up running only a 3-4 gallon nute solution, so the damage wasn't as bad as it would have been had I run a flood-n-drain of the same size off a 30 gallon res!