I have read a Ph of 8 is a good target for tilapia breeding tanks. But, the nitrafication process that processes ammonia drives the PH down by producing acid.
My tap water is liquid rock with a PH around 8. After a week, the breeding tank water is down around 6.5 or less. I worried about dumping PH 8 water into PH 6 water and changing the level too fast. Found crushed oyster shells sold by feed stores for chickens works great. I made mesh bags and put a cup of washed oyster shell flakes in the 20 gallon fry and fingerling tanks. Also put oyster shells in one of the media baskets of the canister filters for the main breeder tank. After a week, the PH is 7.8.
Whether it makes the tilapia harem frisky or not, we will have to see. At least it will keep the tank PH close to the tap water used for water changes.