Here's what I'd recommend:
1. If you are not ALREADY testing with a Taylor based DPD-FAS kit, with fresh (this season) DPD powder and FAS solution, get one from Amazon and use it to test BOTH your pool water AND your tap water.
2. Go ahead and do a bleach bath test, using 1/8 cup of fresh Walmart 10% pool bleach in 25 gallons (FULL fiberglass shower/tub) (~20 ppm) or 1/4 cup in an old full size cast iron tub . . . filled with COOL water. BUT, before you do this, you MUST test your tap water for combined chlorine with an ACCURATE test kit. Some municipalities are treating with monochloramine, which can cause a variety of problems, ranging from lead leaching into the tap water to rashes . . . just like yours! If you detect combined chlorine in your TAP water, you need to do a tap water SOAK before the bleach test, with the bath water adjusted to the same temp as your pool. This Habor Cooking Thermometer 6" Probe works well, and is both accurate and cheap.
3. Assuming that your rash is NOT triggered by a mild bleach bath, you need to eliminate possible sources. Based on your treatment history, you have a witch's brew in your pool. And, since the pool is small, the BEST solution is a drain and refill -- and NOT just because of the CYA. But BE CAREFUL!! Fiberglass pools can float OUT of the ground, if the ground is at all wet.
Before draining, you MUST dig a post hole along side the pool, as close as you can get. Dig as deep as you plan to drain (ideally, all the way -- partial drains are VERY inefficient). Let the hole sit covered overnight. If it's dry in the AM, you can drain, but NO FARTHER then the SAME depth as the bottom of the hole. Also, do not drain if significant rain is forecast. If you can not drain this way, it's possible to do a drain in place, using a tarp that is LARGER than your pool. I'll explain that later, if needed.
4. While the pool is draining and refilling, remove the 'guts' of the Nature2 AND turn OFF the ozonator as well as plugging plug the air intake. (Or, even better, completely remove BOTH units. The ozonator is worse then useless, and the Nature2 is -- at best -- and overpriced trichlor feeder.)
5. Also, buy some sodium dichlor 1# bags [ Sams Club, Amazon #1, #2, #3 ], 3 gallons of Walmart 10% pool bleach, a bottle of dry acid (or muriatic acid, if you can handle it OK) and 2 boxes of 20 Mule Team Borax . . . and NOTHING ELSE. Nada. Not even a little. If you've got other pool chemical stuff squirreled away, GET RID OF IT, so it is not accidentally added to the pool.
Chlorinate with the dichlor, till your stabilizer is ~40 ppm, then switch to bleach. Use the borax to RAISE your pH if it reaches 7.0 or below. Use the dry acid to LOWER the pH if it reaches 8.0 or above. Do NOT use ANY other chemicals.
. . . let me know how it goes, please.
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