Can't maintain free chlorine
Hi Rick,
First off, don't worry about nitrates or phosphates. 99.999% of the time they are nonsense and nothing but pool store hype.
Also, you should NOT be using any form of dry chlorine at this point. Here's why:
Your CH (Calcium) is OK, but much higher--past 400 and you may get scaling. So Calcium Hypochlorite, Cal-Hypo is a BAD idea as it will quickly push you over the limit. BTW, since you have a fiberglass pool, ANY level of CH, from 0 to 400 is FINE--you NEVER need to add calcium, it's only for masonry pools.
Your CYA (Stabilizer) level is fairly high--75ppm. For an SWCG system that's well within the recommended range of 60-80, but you don't want any more CYA. Therefore anything with Dichlor powder or Trichlor tablets is a BAD idea. Dichlor adds 9ppm of CYA for every 10ppm of Free Chlorine (FC). Trichlor adds 6ppm of CYA for every 10ppm of FC.
So the ONLY dry chlorine source left is lithium chlorine, which is insanely expensive.
So you need to switch to bleach or liquid chlorine (LC). LC is merely very strong bleach, usually 12.5% concentration. Those are your ONLY reasonable alternatives.
On to your problem. Despite your pool being clear you are fighting something. Have you added anything else, like algaecides? Polyquat 60%, while being good stuff, drives FC levels way down. But most likely you are fighting something and barely keeping it at bay. Do you have the test for CC (Combined Chloramines) or TC (Total Chlorine)? Since TC = FC + CC, if CC is >0 you may be fighting it. Doesn't matter, because how you treat it's the same.
With a CYA level of 75, you need to raise your pool's FC level to 20ppm. You'll need to start by adding about 5 1/2 to 5 3/4 jugs of 8.25% bleach--regular, unscented, and be prepared to keep adding it at that rate. I'm assuming the jugs are 121 ounce jugs, not full 128 ounce gallons. (each full gallon of 8.25% would add 3.73ppm of FC, but 121oz jugs only add 3.55ppm). If you can find 12.5% LC, each gallon will add 5.7ppm of FC.
You'll also need to check your FC level 2 to 3x / day. If only twice, check it first thing in the morning and in the evening. If it's below 20, add bleach to make up the difference. When your overnight FC only drops by 1 or 2ppm, you're done and can let your pool drop to no less than 4ppm.
If you don't have a test kit with an FAS-DPD chlorine test (NOT the same as a DPD or OTO test), then you'll need an OTO test kit. The best you can buy locally, if you can find it, is the HTH 6-way drop test kit that seems to be exclusively at Walmart. It's OTO test goes to 5, but if you mix 3 parts of steam distilled water with 1 part of pool water, its measure of "5" is really 20ppm.
But you'll need to be persistent both with the testing and the adding of bleach. Run your pump and filter 24/7 until this is cleared up and whatever is metabolizing your chlorine is gone. Meanwhile you should vacuum and brush your pool daily, just to make sure you are knocking any incipient algae off the walls.
You don't need to drain your pool.
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