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Would this be a good Idea? How big of a tank would i need? Would they get along?

I wouldn't do it. RES are pretty territorial, and will likely exploit the ambo's comparatively poor swimming abilities to attack. They are from completely different habitats, and their optimal water depths and temperatures are different (temperatures, not so much).

If you decide to get the box turtle, I would encourage you to keep it separately from the RES.

If you're still curious about tank sizes, an adult male RES needs at least a 70 gallon tank. An adult female will need at least 110-125 gallons.

I wouldn't suggest anything smaller than a 30 gallon "breeder" (36X18X16) tank for the box turtle. They don't do that well in deep water, so a wider tank is better than a tall one.

Ok that note my red ear silder is as big as one ounce of silver coin. So should I get a larger box turtle Then? Or would that be a still a bad move?

If you do keep them together, there's still the off chance that when they grow bigger there will be problems, and it's going to be a financial problem down the road to get another tank later. That being said I still don't think it's a good idea to put them together now even though I know nothing of the Box Turtle.

Am thinking when my red ear sidler gets too big am moving it to turtle pond am planning on making. Also thinking about buying a 75 gallon in few weeks to get really for the box turtle.