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If gravel is an unacceptable substrate then I will use river rocks...

Though how am I supposed to clean around and under?

Would a simple glass bottom be a better solution?

What about planting substrate and carpeting the tank with plants?

Thanks for looking

some people have plain glass and some have sand
its all your choice

I just have a few river rocks in the bottom of my tank. I never really have to clean under them because my turtle is always moving them and shaking up any debris under it so the filter can trap it.
If you have a few rocks, it's not a serious debris issue. It's when you have a full tank of sand or river rocks that you get a lot of buildup.

Theres no "buildup" when you have sand, it all sits on top.

I have sand, love it.

Do you siphon the sand to remove debris?

Only when I do a water change. Other than that, I let the filter take care of it, or occasionally use a net, or the zoo med sand sifter.

If you have a full bed of river rock, you'll have to move them around and use some sort of siphon to pull the waste up. You'll have to do this much more often than if you have a bare bottom tank or sand. I have sand in mine and I clean it once every two weeks or so. The whole process takes me max of fifteen minutes. Bare bottomed is the easiest to maintain though.

I had planned on using the Python system to wash the gravel, if river rock will be too heavy I would rather not use it to bed the bottom. I would like something, though, I have used a bare bottomed tank for practical purposes but feel I would like something better looking now.
Also, the turtles seem to like exploring, and if a varied surface keeps them occupied then maybe they wont get territorial?

Would anyone recommend using sand with scattered large rocks, and if so a type of sand?

Would anyone recommend using soil (planting substrait) and attemting a planted tank? I've heard turtles like to dig and rip up plants even when not hungry.

I would not recommend soil. But I have fifty pounds of play sand from wal-mart, about two dozen scattered river rocks, and about a dozen fake plants in my 125 gallon tank.

sand is like...amazingness times ten.
i highly suggest you get it.
like chris said, there's no build up.
when theres debris, you either let the filter get it
or you can just sift it out.

i had river rocks and it trapped A LOT of debris.

will the python siphon vacum pull hard enough to drain the sand or will it stay like gravel?

just get your sand from OSH or home depot.. Quikrete play sand or commercial grade. I have some river rocks randomly placed throught my tank on the sand, and they look nice.

Yes it will if you put the siphon close enough to the sand.

My canister filtration is in my sump, I am relying on a return line spraybar to distribute flow and keep waste suspened to either go into overflow box or settle on far side to be removed with vacuum siphon.

Reading posts about RES eating sand shows that I will first need a test container of it in tank.

Thx for the advice, I have thinking to do.