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I currently have a 55-gallon bow front aquarium with an adult RES, and a Tetra decorative in-tank filter for up to 55 gallons. The water level prescribed by the filter is 4-6 inches, and I have it right to the top of the six-inch mark. The filter is working fine so far and my turtle uses it as a basking spot (with the help of another rock as a stepping stone).

Still, it seems like a lot of water space in the tank, and I would like to make the water deeper. Obviously just adding water would submerge the filter and negate its usefulness, but what if I put something under the filter? If the filter was sitting a few inches off the bottom of the tank, I could add that much,more

That makes sense. I would say,"go for it!"

However, I would also say that the filter is underpowered for a 4 - 6" RES. Ultimately, you want to look for a larger canister filter.

Filters are rated for an aquarium with fish. The waste fish produce vs the waste a turtle produces is huge. You really want a filter that is designed to handle many more gallons than you really have to deal with the waste your turtle produces.

Sorry for not being more specific; it's a Tetra Repto-Filter that is designed for shallow tanks with reptiles and turtles. Is the 55-gallon recommendation a standard for a fish aquarium regardless? Also, although it's a 55-gallon tank, with only six inches of water in the bottom it's probably more like 20 gallons of water. Raising the filter it as I described would increase that to 30 gallons, I would estimate. Is my filter still not powerful enough?

No, it's not really powerful enough.