The 183 is a blackwater species. Because few people actually keep them in this sort of water, many do not do well in captivity.
Yeah I've come across that with these guys before. The particular line i'm working with was tank bred and him and his parents were both kept in water conditions very similar to mine. (part of the reason i'm not TRUE blackwater.) I've tried to find a happy medium between both their water conditions and they both seem to have adapted fairly well to it, in fact the L204 (from what i can understand) solicits for a female almost continuously (fastidiously cleans the cave, slowly swings his tail back and forth in the mouth of the cave displaying it, and displaying himself in and around the mouth of the cave during day time, as well as choosing a specific territory to care for and defend.) So, its certainly a learning experience for me and one that i'm prepared to adjust to as time passes.Not likely the 204 will be bothered by the 183, they don't compete for food. One thing though, unlike most SA plecos the 204 actually comes from hard, alkaline water. Keeping it in soft/acidic may shorten its life span, I'm not sure how wide their adaptability range is.