Made a HUGE rookie mistake, almost disaster!

I'm not concerned about my tank overflowing, the return section of the sump only holds maybe 2 gallons, and the ATO holds 3 gallons, so worst case scenario is the DT has an extra 5 gallons in it. It can hold that without overflowing.

The problem arose when the ATO emptied itself, and then I proceeded to add more without realizing there was a problem. It was my dumb arse that almost caused a miniflood, not the tank's.

 

darn snails been there done that my new tank is being set up for 1 1/2 drains 2 in total not becose i need it redundancy always wins in a flooding disaster not to mention snails should pass right through and end up in the sump