I agree with the others above, a 55 will eventually be too small for a red devil. A 75 would be the absolute minimum, permanent home, reason being the 12" width of the 55 is not enough for a fish which can potentially reach 12+" itself.
Here is a two year old Saggitae, cousin of the RD, in a 125. In that 55gal his nose would be on the front glass, tail touching the back....
And we have not even mentioned that a fish that size in a 55 would need fin level water changes daily.
You can use it to grow out for a time, till about 6in. I would then upgrade to a 75 as a min for a single fish. Or a 125 for multiple fish.
Petsmart has 125s on sale for $499 at times. I got mine for $472 with rewards