You can keep a trichlor tab in the skimmer., but trichlor is acidic and if you want to bring up your ph it will be more difficult with it in. I never used oxallic acid, I've only used the ascorbic acid. The reason you would want to use bleach, is the ascorbic acid, and maybe also the oxallic acid eats up the chlorine very quickly. You would have to put a lot of pucks in the skimmer to get the chlorine up, and that isn't a good idea. You can just add the bleach about 1 gal at a time, take out the puck if you add bleach through the skimmer, or you can add bleach to the pool and leave the puck in. Trichlor will also bring your ph down, and it is hard to get your ph up sometimes after the stain treatment. You can also use the cal-hypo granules, but watch if you have high calcium. Just never mix different kinds of chlorine together - you can add them to the pool just not together, in the pool they are diluted. The reason you want to add the chlorine slowly is because if there isn't enough sequesterant in the water, the metal will fall out and stain again. Just keep adding the chlorine till you get the chlorine to 3ppms. since you have a cya of 25. Keep it on the low side for at least 2 weeks, and don't shock the pool for at least 2 weeks.