It is entirely possible that the backwashing you did actually washed out the CYA you added before it had a chance to completely dissolve, since it can take up to a week to fully dissolve and register in your water, and it is also probable that frequent backwashings have lowered your levels some. That may well be why you were having trouble holding chlorine in the water, especially coupled with an impending algae bloom that finally caught up with you.

What do you use for chlorination? If powder, is it cal-hypo or dichlor? Do you use pucks? I'm guessing it's cal-hypo by your reported Calcium level. Is your CYA typically high at the end of the season, and low or non existent at the beginning of the next season? If so, then the CYA is probably being biodegraded during the winter into ammonia, which can create quite a huge chlorine demand when the pool is first opened. Many of our posters here experience this, and for some reason it seems to be increasingly common over the past couple of years.

When you shock the pool, you need to get it up to the correct chlorine level and hold it there until the water clears, and until you're not losing any chlorine overnight--if you can test after sundown and again before the sun is on the pool and don't lose any chlorine during that time, then you're ready to let it drop back down. If you are losing chlorine overnight, then you know it's due to stuff in the pool needing to be killed and not due to the sun--so you keep that chlorine up until you don't lose any overnight.

So...in your case, I would get it back up to shock level (based on the CYA of 20 would be 12-15 ppm) and keep it there by testing and adding more chlorine as needed to get back up above 12 ppm, pump running 24/7 and backwashing filter as pressure indicates, until it clears. You might find it easier to use plain, unscented bleach for this purpose, which won't raise your calcium levels any higher. Once you get it cleared up, then I would test your pool a couple of times a day and see if the stabilizer just needs to be bumped up a little further to make sure you have adequate chlorine in the pool all day so you don't have another bloom. Most folks around this forum try to target 30-40 ppm or so, but I wouldn't worry about that until you get it cleared up.

Janet