Alkalinity High, pH Low or Normal

Hello. Newbie here. I have a 25,000 gallon, inground, vinyl rectangular pool, with 2 returns and 1 skimmer and 1 main drain. I use a solar cover whenever the pool is not in use. I use 3" slow disolving chloring tabs in an adjustable Hayward chlorine feeder. Filter is diatomaceous earth.
I've had the pool for 22 years. About 5 years ago I was having trouble keeping the chlorine level up. Did my research and have tried my best to keep the pH level good, which is primary for keeping the chlorine up. It worked well. I noticed that I had to add pH up frequently to keep the pH at a good level. Since I have been doing this I have rarely had to add Alkalinity Up. It was always up, at around 120 which is acceptable. But this season, I have still had to add pH up at times, but this year my Alkalinity has been going up. Todays numbers are: Chlorine a bit low. pH 7.5. Alkalinity 160. I think its time to attend to the Alkalinity.
What are your suggestions to lower the Alkalinity, without dropping the pH, which would result in me adding pH Up, which would raise the Alkalinity. I've been reading about adding acid to the deep end, away from the return, with the main drain off, but then I read that doing this might melt my liner. Remember, it is VINYL.
Any help you could give me would be appreciated. Thanks, Dave