You don't need all those products and you need to stop using the ph adjuster. Your tank falling to the 6 ph range is causing your tank to have to recycle. You need to test your tap water. In order to get a proper tap water ph reading you have to put some tap water in a glass and either aerate it or stir it off and on for 24 hours. This allows the water to gass off and after 24 hours you will get your tap waters true ph. Also you need to get an API Master test kit as strips are known for not being the most accurate.
If you can't keep your ph up then you need to consider adding some crushed coral to your filter or add some to a media bag and hang it under your filter outflow. Start with a little CC and see how much it raises the kh
/ph after a few days. Then add or remove CC as needed until you get the level you want in your tank. I also suggest getting an API Gh
test kit. It's two tests in one box. As long as your Kh
if above 4 on the API test you have enough buffers in your tank to keep your Ph stable.
You also need to do a WC
anytime your ammonia or nitrite rise above .25ppm. Once the tank is cycled and you stabilize your ph you just need to go to doing 50% WC
That is a common plecostomus which will grow way too large and have too high a bio-load for a 20g tank. You need to rehome him.