The pH isn't a problem, as long as it's stable. Fish that you buy locally should be adapted to local water pH. We have 7.6 pH here.
The red gills are a sign of ammonia/nitrite poisoning. If you're not using the API liquid tests, I'd recommend their master
FW kit for accurate results. Usually nitrite means more purplish gills but the nitrates could be an issue too if they are over 10-20.
You should start using Prime for the water conditioner - it will help with locking those toxins. If you don't have aeration in the tank, I'd recommend that, especially for the corys. Also I'd do about 25% water changes every other day, testing the water before each
water change, until the ammonia & nitrites are 0 and the nitrates at most 20, but preferrably 10 or less.
Do you have a pic of the pleco? The plecos that you have - are they common plecos? If so, you'll want to find them homes when you can. A 75g would be small for 2 common plecos and if they're older, they may already be experiencing growth stunting. Does he act normally or show any other signs of something wrong?