If you only drain a foot and refill that won't make much of a difference to the CYA and you will have to repeat the process many times which is not very effective. I would bite the bullet and drain at least 3/4 of the pool. Make sure your water table is not high as you said you have a glass pool. 7,500 gallons should not cost you that much money. Like the above posters said use bleach from now on.