Sorry can’t say I’ve had a similar experience with RGs.
However, if this were my fish and it’s eating well and doesn’t seem to have swimming issues, I would just continue to focus on good husbandry practices and see if things improve.
If it is indeed fluid accumulation from edema then as Jay Hemdal mentioned this could signal kidney or liver issues and would be difficult if not impossible to treat. Close observation of the RG may be necessary to catch any secondary infections early on since its immune system would likely be compromised.
Also, I noticed you said yours is a low nutrient system with no skimmer and you seem to feed a decent amount of food, do you have any algae issues or issues with Dino’s? If you have algae issues it could be that your nutrients read 0 because the algae is up taking it all. On the other hand, If you really do have no nutrients Dinos are a risk. And being planktivores, your RG may have consumed some Dinos which would be toxic over time.
This is an obvious one but check your parameters and make sure nothing is too far off. If anything does seem off, bring the parameters back within range gradually, not overnight. Even if the RG is too sick to save, at the very least getting your parameters in check should benefit the other fish in your tank. Hope that came across okay, not trying to assume or accuse you of bad husbandry, just trying to understand your tank conditions!