I would recommend checking your nitrite levels. Nitrites can cause irritation and flashing in the absence of any obvious parasite infections, and at very high levels, can cause skin and gill damage. Pbass eat up a lot of protein, which has a lot of nitrogen in it, which produces a lot more ammonia in their waste compared to other similarly sized fish species. Most of the bacterial additives for tanks are nitrifying bacteria, and may not have enough of the right species that would further reduce nitrites to nitrates -- but those come with their own set of problems.
Keep up with those water changes!