Where did my CYA go and when do I cover my pool?

Hurrah, my first real post here on PF! Got my Taylor 2600 test kit today, and here are my first results:

FC: 2.5
CC: 0
PH: < 7.0 (Base Demand = 6 drops to 7.4)
TA: 130
CH: 60
CYA: 25

I'm a beginner with a 3700 gal intex with SWCG, and am starting to figure things out with the BBB method. My first question is, what happened to my CYA? I had it up to ~90 about a month ago, and now it's down to ~25. In the interim I've replaced perhaps 300 gal water due to various losses. Is it normal for CYA to diminish that much?

Next, exposure to sunlight is essential to breakdown of "goo," so when am I supposed to use this opaque vinyl pool cover I spontaneously purchased? At night only? Or should I just put it in the yard sale? I run my SWCG for about 5 hours at night starting around dusk.

I know I need to get my PH up. Any other tips?

Thanks!

Replacing 300 gallons of water of a 3700 gallon total is quite a bit of water, and it's no surprise that your CYA has diminished...especially when you add in water lost due to splashout, backwashing, etc. However, check your owner's manual for the SWCG unit-they normally recommend the CYA level that your unit will operate best at--and that recommendation is designed to help the unit work at its most efficient and make the cell last as long as possible. With your CYA that low, you're over-working the unit (or underchlorinating your pool).

You do need to get the pH up, and depending on how low it actually is, that can be critical, especially if you have a heater or vinyl liner. We recommend Borax for that purpose. Sorry I can't help you with the pool cover question, but I'm sure someone will be along shortly that can offer you some advice on that.