Hi. There's no such thing as "Chlorine Lock"

OK, you want to get your chlorine level up to 25ppm and keep it there! For the moment you can guestimate: add 2 1/2 gallons of 6% regular unscented bleach for every 10,000 gallons of water (that will add 15ppm of FC). So if you have 20,000 gallons add 5 gallons of bleach. If you have 15,000 gallons, add 3 3/4 gallons (ah, just add 4 gallons!). You'll need to have your water tested every day. Keep FC at 25 till the water clears.

You'll want a proper test kit. Click on the "Amazon Links..." in Watermom's signature.

Run your filter 24/7, brush and vacuum your pool to waste every day.

I agree with 'mom--a CYA level of 300ppm is probably a mis-reading, but if you've been using vast amounts of chlorine tablets, having well over 150 is almost certain--and the effects sound right.

Tablets add phenomenal amounts of CYA and people who aren't taught to test their water weekly for CYA levels get into precisely your situation. One must stop using tablets when the CYA level reaches or exceeds a good target level, usually 50ppm (though we have situations where more is preferable).

Stabilizer is a two-edged sword. It helps protect and preserve chlorine by...making it act slower and therefore, less effective.

Carl