Turtle Tank 50 Gallon - Trickle Tower Filter

Hi i've been doing a lot of research on building a trickle tower filter. However i cannot find out what size pump is required or the size of the tank the tower needs to be in so no over flowing occurs if theres a power outage or the pump fails, for roughly a 50gallon tank assuming around 30 gallons of water.

Also any pros or cons for usings the following media:
Top Level:
Filter Floss
Mid Levels:
BioBalls
Plastic Pot Scrubbies (BioBalls)
Lower Level
Lava rock

I've also seen two styles of the over flow:
1. A u-bent overflow inside the tank with a cap with drilled holes to intake the water to the tower.
2. A 2" chamber capped at one end, with a 3/4"pvc running almost to the bottom of the chamber, and then one more cap with a 3/4" hole for the pvc and also small holes around to let the chamber fill up with water and then the 3/4"pvc is used to intake water into the tower. Only issue i see with this: none of the degree on the top of the water will ever get filtered or will gravity/physics play a roll with that?

I plan on having the return on the opposite side of the over flow and also above the water line to create bubbles/oxygen for the tank and also keep good circulation.

If anyone has any advice or experience with trickle towers, I would greatly appreciate any comments!

I'm leaning towards a 10-20gallon tank/tub, heater in the sump area, with a (3) drawer system and a 500/600 gph pump. I also read the pvc sizes for the water coming into the tower and pvc returning water back into the tank need to be different sizes; the pvc returning water back into the tank needs to the double the size as the pvc to the tower...? Only saw this come up once... Any thoughts?

Thank you very much, i know the turtles will appreciate it even more!

debris* not degree.... sorry about that! Also i'm trying to make a very efficient trickle tower filter at a reasonable price.