Hello and welcome to the forum. Sounds like you are seeing the problem that trichlor tablets create. If you have a short swim season, or your pool gets a lot of water/snow during the winter to dilute CYA, or you backwash your filter frequently (and water is cheap and plentiful) then using tablets may be OK. But a great many pools are NOT so diluted, backwashed, or short seasoned enough to manage on tablets all the time.
Here at TFP we know that when you open your pool, you really need to accurately test the water to see WHAT it needs.
It may be that the CYA you had last year is still there. It may be that it has all gone away. If you add liquid chlorine and CYA is very high, it may not be enough to actually sanitize the pool. If you add liquid chlorine and CYA is very low, it could damage your liner, for example, or simply be wasted as it decays due to sun exposure without the protection of stabilizer in the pool.
Without a good water test you are flying blind.
As regard shocking weekly, if you keep the chlorine level stable by testing and adding smaller amounts more often, you will not need to shock the pool. Many of us rarely shock the pool. I think I did it once about this time last year. I did it once when some vultures left something nasty in the hot tub. I did it once when I found a dead baby squirrel in the pool. So, about once a year on average and that is in response to a specific problem, or sometimes it is due to my own general laziness letting the chlorine go to zero early in the spring when I am not used to monitoring the pool regularly. (Here in TX I never close the pool, but do slack off to testing every few weeks in winter and I have to remember to get back to regular testing by May.)
If you read Pool School, you will grasp the chemical interactions that are going on in your pool. Regular testing will tell you what is changing and the Pool Calculator will tell you what you need to add to the pool.
While trichlor tablets are EASY, they are a luxury that some of us cannot afford due to the constant build up of stabilizer. In my pool, tablets will over-stabilize my pool in just 2 weeks of constant use. I have to use liquid chlorine most of the time to avoid that.
I've already taken a 2 week trip this spring and so while the CYA level was quite low from our wet winter, 2 weeks of tablets has put it at the top of the safe range now. Any later vacation will likely not be so easy for the pool sitter as he will have to test and use bleach 3x/wk instead of just loading the auto feeder once a week. If it keeps raining like it has, maybe CYA will decline some by the next trip. The testing will tell. In the meanwhile, I use jugs of bleach or liquid shock, and muriatic acid as needed based on testing every other day or so.
