stop adding it to the tank....I am sure others here would agree. I would just put that stuff on a shelf for a while and get back to basics. Feed them the food as it is packaged as see what happens for a few weeks, hopefully a water change will get rid of some of that garlic . Also the ph of liquid garlic is around 6.0 to 6.4 and sometimes as low as 3.7 or less. So this may also be adding to you ph issue.
When people use garlic, they just put a drop on the food and let it sit for a ten minutes or so, then feed the fish. Again, I hope this helps and I am just offering suggestions to what I have learned through the years from other on this forum and others...