Just to explain a little further: contrary to what the pool industry generally believes, cyanuric acid can be bio-degraded (eaten!) by bacteria. The bacteria can then 'poop' one of three things: nitrogen gas, which causes no problems; nitrates, which causes no immediate problem, but fertilizes algae later; and ammonia, which is a huge problem, so much that you could think of it as 'negative chlorine'.

Depending on how the bacteria have gone about things, it make take nearly 1.5 ppm of chlorine to clean up the residue from each ppm of CYA that's missing from your pool. That may not sound too bad, but what it means is that if you had 70 ppm of CYA last fall (pretty common) and none now, you may have to add 100 ppm of chlorine to your pool, before you are able to maintain a residual.

Until you have done so, you will either have to (a) drain and refill your pool or (b) add dose after dose of chlorine, till the ammonia is gone.

Just a reminder: draining your pool can be very dangerous to your pool. Vinyl liners are often destroyed when inground pools are drained; if the ground is wet, concrete and especially fiberglass pools can literally float out of the ground, when drained , and so forth. You'd better be sure it's safe before your start.

Most people are left with the need to clean up their pool water as it sits.