I need help with my african cichlids

I have a 37 gallon tank with 11 cichlids and 1 pleco. I have recently lost 4 cichlids and the pleco, so I took the water to the fish store to be tested, and they told me the ph was too low, it was 6.6 and it should be around 7.6 at least for african cichlids. It also started growing a really bright green algae really quickly after the last water change. I don't know why the ph has dropped, I've been using the same tap water that I always have. How can I bring the ph back up naturally with out using chemical buffers?

I would do some 25% water changes every other day for 10 days to see if that brings it up before I started putting things into the tank that will change the water chemistry. Test your tap water and see if the water company has made some radical changes they haven't sent out notices about.

"Re-cycling" does NOT affect the pH. Have you recently installed a water softener or added new salt to an old water softener. Salt LOWERS pH if you add too much.

If it has been a looonnnggg time since your last water change, that is the reason. Build ups of ammonia, nitrites and nitrates over weeks or months without weekly partial (25%) water changes will lower the pH. If you just did a large water change after not doing one for a long time, that could have killed the fish due to the large change in pH and other parameters.

It is amazing what fish can live in if it builds up slowly over a period of weeks or months. Then fresh water actually kills them because of the shock of change.