How did you get a cya reading of 150? I don't think any of the testers differentiate anything higher than 100.

So, apparently it is at least 100? That is WAY HiGH for cya. If you can't afford to do a partial drain and refill, then you will have to run high chlorine levels to compensate for the high cya. Take a look at this chart:

Stabilizer . . . . . . Min. FC . . . . Max FC . . . 'Shock' FC
=> 0 ppm . . . . . . . 1 ppm . . . . . 3 ppm . . . . 10 ppm
=> 10 - 20 ppm . . . . 2 ppm . . . . . 5 ppm . . . . 12 ppm
=> 30 - 50 ppm . . . . 3 ppm . . . . . 6 ppm . . . . 15 ppm
=> 60 - 90 ppm . . . . 5 ppm . . . . . 10 ppm . . .. 20 ppm
=> 100 - 200 ppm . . . 8 ppm . . . . . 15 ppm . . .. 25 ppm

This means that you will have to be diligent about keeping your chlorine level between 8-15 ALL the time or you will risk an algae bloom. Also, in the event that you do need to shock the pool, you will have to take it up to 25ppm. Actually, since the chlorine level has dropped to 0, there is a good chance that you have something brewing in your water and just can't see it yet. I would suggest going ahead and shocking this pool to 25. If you test in the evening and don't lose more than 1ppm of cl from sundown to sunup, then you are ok. But, if you do lose more than that, shock it up again.

If you decide not to do a partial drain and refill, you will have to have a good test kit to be able to test the high cl levels you'll have to keep in your pool.