Also, every 'shock' product is primarily some form of chlorine, whether liquid or powder, with additives you may or may not want in your pool. Powder costs much more.
Chlorine is bleach, and is available at different concentrations. Higher = stronger.
You personally can't keep chlorine in your particular pool because most likely it is being used up by killing the algae.
Algae can make your pool cloudy, too. And if not killed, it will grow make your pool green.
Right now, the algae is growing just as fast, or faster than you are killing it. That's likely another reason why you're not seeing improvement.
In a virtually unused, perfectly clean and chemically balanced pool, a Free Chlorine level of 3 is pretty much the absolute minimum. A Free Chlorine level of 5 is much better.
And you post that you are at .2. Are you sure it's not 2 rather than .2?
If it is .2, that's 1/25th of the amount of chlorine required to maintain.
If it's really 2, that's less than half necessary to maintain a clean and used pool, and as the level for killing algae is 15, approximately, you are around 1/8th the amount of chlorine required for the shock process.
Please Read Pool School. Several times. You might start with The Shock Process.
Don't let them sell you products for Phosphates. While it's true that algae feeds on phosphates and other 'debris' in your pool, if you kill the algae, it won't be there, eating and growing at all.
You can keep your pantry full of food, but if no one is home to eat it, it just stays there. Right? Same thing with phosphates in your pool.