According to IGFA the average sized 'rainbow bass' Parachromis dovii, caught are between 5-10 lbs, but the first IGFA world record guapote was actually caught in Costa Rica in 1991 @ 12 lbs 9 oz, only then was broken in 1999 by an angler in Southern Nicaragua @ 15 lb 0 oz. Next largest (recorded) since then was in 2008 @ 13lbs 80z. ALL were well under 30" being closer to 26". Two Alleged 26lb & 31lb both @ 32" monsters were 'said' to have been caught. Both however, by unknown method, and no photo or physical proof, or name of the fisherman was ever provided. so it is considered hearsay and a fishermans tale. Possible? maybe.. however dovii is a VERY popular sport and food fish in Nicaragua and Costa Rica so considering the documented records on a fish is 10+ lbs less and quite a bit shorter in size than the proven fish, on a fish that is commonly fished for I would take the two that are are undocumented off the table.
can a 30" exist? I would think probably. if the 3 known world records are 12, 13, & 15 lbs all 3 near 26" I could see a possible 18lber @ 30" being probable. however, none have yet to be caught or seen so we wait. as for the alleged 31lb, 32" tale... man that would be a sight to behold!!
Keep in mind as mentioned above, these is a very commonly fished species for food source. In fact they are prohibited for collection and export in Nicaragua(why we have no wild ones in the hobby) in the big lakes(river populations are ok), it is certainly plausible that the locals fishing for food have caught larger. They just go undocumented and are eaten