Small water changes are better versus large 40+% changes because it changes the waters chemistry very little. You are doing 15% pwc
weekly now but at 5% 3 times a week. IMO
that's more work then it's worth.
Larger 15% pwc
weekly won't harm stock as long as you closely match the ph/sg
/temp to the main. Plus you only have to do it once a week
Personally I use a 30 gal
clean trashcan to store/mix my pwc
water. I run a Mag 9.5 pump
in it to mix the salt and the pump also heats the water to 79 degrees constantly so I don't need a heater. When it's time to do my weekly pwc
I drain 10-12 gal
to the toilet and then hook up a hose to the pump to pump the new water into the tank. The whole process takes less then 10 minutes and is mess free.
One other note is I wouldn't add the buffer to the tank directly and either mix small amounts (1/4 teaspoon) to your fresh top off water or a little more to your pwc
mix. How many teaspoons have you added so far in total?
7.8 ph (especially in the morning) isn't drastically low and more then likely is the result of the salt mix you are using. Over the next couple of weeks you should see it slightly higher. Even if it stays around the 7.8-8.0 level any fish/inverts you add to the tank will adjust once you drip acclimate them.
If your tank is already cycled I'd add small amounts of food (nh3
source) to keep your bacteria from dieing off before you add fish/inverts.