I have a small 250 gal. pre form pond & I too have had success with using peroxide to keep the algae growth down. I use half a quart bottle of that WalMart 3% &dump it near where the water comes out of the Skippy filter (& make sure there are no fish directly where I dump!). I don't want the peroxide to kill the mint I have growing in the Skippy. I read to use this somewhere on this forum & it sure seems to work! I empty the filter in winter & toss the mint but I always have more in the herb bed to use the next year! I put the peroxide after it comes out of the filter as I didn't want to harm the filter/mint system in the Skippy. Have had clear water now for the past two years & only the short green stuff that clings to the pond sides that the fish love to nibble at. Thank you Jill for verifying the use of peroxide!!

OK, Thanks for the input. The pools drain down to a depth of around 2 inches and fill to about 5 inches over a 10 to 15 minute intervals depending on the total amount of water in the system. It will operate with as little as 45 gallons total, but the cycle time is around 15-20 minutes. When there is around 100 gallons in the system, it cycles in less than 10 minutes. Perlite is the main growing medium in use. Some pots filled with a fast draining potting mix sit in a lower pot filled with perlite and act as a wick. Other pots have plants growing in perlite. This system was built as an exeriment to see if a single system can be used to grow both hydroponic and soil based plants. So far, the piggy-back soil based pots that sit in perlite filled pots seem to be doing fine. So far, after around 3 weeks, there has not been any evidence of problems of too much water in the soil to adversly affect growth and production. The plants that are growing in the perlite are also doing well.

There are extended test kit directions as a sticky on Trouble Free Pools site that will show you how to dilute your pool water by half before testing your CYA (then you multiply the test result by two to get back to your "real" CYA) if you have your own test kit - this might work with the pool store's testing (I'd do my dilution before I take it in and not mention it to the store - they often like to argue with me about TFP and BBB and the rest). I ordered my own test kit and got it within a few days and am so happy I don't have to rely on the pool store's guesses on my CYA now, so would highly recommend that. Your target CYA is different if you have a salt or regular chlorine pool, but you can plug numbers into the website the pool calculator dot com and see how much it recommends you drain. However, it's only as accurate as to the numbers you can provide, so getting a solid CYA first would be helpful. I plugged your 23000 gal pool in, and asked it to take a CYA of 180 down to 50 (where I try keep my non-salt, full-blazing-sun pool with BBB method) and it says you need to drain and refill 72% of your water. I would not do that all at once, I would drain some and refill, drain and refill, in several cycles. Pools have a way of popping out of the ground if you don't keep water in 'em. There are several threads on TFP related to water replacement due to CYA, and lots of helpful folks to give advice. Also of note - to a certain extent, my understanding is not necessarily that your chlorine is ineffective at high CYAs, but it's more difficult to keep the chlorine high enough to be effective at the high CYA levels, but I am science challenged so I don't know for sure if that's exactly right. I do know, the higher the CYA, the higher your Free Chlorine must be. There is a chart showing the relationship on TFP as well, it goes to CYA 100 and says FC must be at 39ppm to be shocking the pool. If your CYA is 200, your FC might need to be around 80 to properly shock the pool, which might be nearly impossible. Pool School will tell you how to properly go through the shock process once you have the CYA under control. Good luck!

Barco is correct about the SWG, super-chlorinate is not going to get you anywhere close to the shock levels you need to get this under control. Black algae is the worst to get rid of because it roots into the plaster. When you scrub it off basically you're just removing the head, a small part of it remains in the plaster and it just comes right back again when it gets a chance. The key is raising your free chlorine to shock level and holding it there while you brush 3 or 4 times a day with a pool brush. Once it's gone hold the FC level for a day or 2 just to make sure. You can refer to Ben's Best Guess chart to determine the shock level you need (based on your CYA level): http://www.poolforum.com/pf2/showthread.php?t=365 And refer to Jason's Pool Calc to determine how much bleach you need to add to get there: http://www.poolcalculator.com/ Don't buy a bunch of "magic cures" from the pool stores, plain old bleach will do the trick and cost a lot less!