Don't use any liquid products to bring your pH down. This is a battle almost every aquarist tries and looses. In the end those products cause more damage then help. Most use a form of phosphate that builds and causes horrible algae trouble, some crash the bio-filter. If your pH is acidic out of the tap (test your tap water , right out of the faucet) then gets more base in those 2 days this is common. Sometimes tap water,either from a private well or city supplied, has trapped CO2 in it. The CO2 will make the water acidic, after a few days the CO2 releases and the water becomes its "true" pH. In this case you should "age" the water for a few days or fill a bucket with water for your PWC and use a spare airstone to aerate it for an hour more or less. It depends on the ammount of water and surface agitation from the airstone. Is this a new tank? If it is an established tank (over a year) look for a possible failing bio-filter from products or too large of fish load. If you are underfiltered or too little water volume, ammonia levels might fluctuate and cause a pH crash. When the filter catches up it "may" get back to normal. If that is the case you may be in store for trouble down the line.

Running crushed coral in your filter will help bring up your kH and stabilize your pH. pH swings and crashes can be deadly to fish.

Crushed coral will also raise pH to 7.8 to 8.0 ppm. If this is not desireable then don't us it. The loaches should be OK with the higher pH. The ideal range is between 5.5 -7.5. I don't think so much pH fluctuations are the culprit to shock or death as total dissolved solids (TDS). pH can fluctuate constantly, and does, with day and night, or when your lights are on if they are high output. Clown loaches are a staple in large planted aquariums because of the snail issues. Most planted tanks fluctuate pH because of the intense lighting. My own have dropped and risen pH from 7.8 to 6.6 overnight with no ill effects on many types of fish. Since I have compressed CO2 injection with controlers or timers I have alot less severe fluctuations. There is no doubt that as great as 1.2ppm would cause stress but that was once in a while. If the fish are otherwise healthy, minor fluctuations should be less impact. pH and TDS are related and there will be more TDS with acidic water because certain solids will break down in acidic water. This is where I think the confusion is.

The question is why the ph is changing. Are you using fertilizer for plants? How about water changes? What are the other water parameters [ammo, nitrate, etc]? Is this an established tank? What is the range of the ph? A big sudden change [1 or 2] is usually fatal. If your tapwater is an OK ph you shouldn't need chemicals to buffer ph IF your filter is adequate and you do proper water changes.