I used a 5 gallon bucket but found I was refilling it every 2 days. Now I use a 70 gallon tank

I used a rubbermaid 25g ish with the tunze ATO. I put in the closet near my tank, about 8' away. Drilled a small hole in the floor ran it on the basement open celing and back up through the heat duct behind the tank. Worked well and hid the res. I had the same problem, never wanted the tub sitting visible by the tank.

Well I got her going and it Just fit into the closet...
Ill probably try and put a 2 foot extention on the hose and wire for the pump but we will see...
Now I Have to find a 30 gal plus container...

Also...
Can the tunze pump run dry or does it have a built in sensor?

It can run dry.

Great ATO system though. It's one of the few pieces of equipment on my tank I completely trust.

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It can run dry but it will turn off after about ten minutes of run time.

Ya, that's how I know that the bucket is empty if I hear the pump running dry (it's rather loud and difficult to ignore).

Sometimes my wife will hear it while I'm at work and fill it up for me.