Successful treatment of "pink algae"

[waterbear]

It is NOT more effecient at oxidizing organics. Monochloramine is a less potent oxidixer than hypochlorous acid. What happens is twofold. First and formost, monchloramine is a nitrogen containing compound and as such is a primary food for algae and many bacteria. It is actaully consumed as such and the "attached" chlorine kills the algae or bacteria from within. The second thing that happens is that the ammonia competes with the cyanuric acid in the water for the chorine and in this way 'deactivates' some of the CYA temporarily so the chlorine becomes more 'active' or effective at killing the algae or bacteria. This is also why sodium bromide becomes very effective since it essentially take the CYA out of the loop, so to speak since the chlorinated isocyanurates, which are the primary form of chlorine in a pool with high cyanuric acid levels readily oxidize the bromide ions to hypobromous acid which is much more potent than the clorinated isocyanurates.
These types of products are much more effective when the cyanuric acid levels in the pool are high and not as effective when they are in more normal ranges. Also, these types of bacterial growth do not occur in chlorine pools when the cyanuric acid levels are normal since more of the chlorine exists in the form of hypochlorous acid and not as chlorinated isocyanurates. Remember that monochloramine IS a chloramine and is not considered to be a desirable form of chlorine in the pool and is, in fact, one of the combined chlorinamines that we test for and eliminate by shocking the pool. Why? Because they are NOT as effective sanitizers or oxidizers as hypochlorous acid BUT when there is a high level of cyanuric acid in the water there is a very low level of hypochlorous acid. Most of the chlorine is in the form of chlorinated isocyanurates unless additional hypochlorous acid well over what is normally needed is introduced to move the equalibrium point of the reaction. (In other words, if your CYA is high you need to run MUCH higher FC levels and shock to much higher levels to have the same activity that you would have with CYA in the 30-50 ppm range and FC in the 3-5 ppm range.

As to why this occured in the third year of using trichlor the reason is simple and had NOTHING to do with your solar cover. Using stabilized chlorine causes the CYA to rise over time and the pool becomes overstabilized. I am interested at what your CYA level actually is. Would you post a full set of test results? Also, what kind of filter do you have. I believe in another thread you said a DE filter but is it a 'bump' flter or a backwashing filter? Remember that it got higher as each year passed and that third year is when it reached 'critical mass' and you developed the pink slime. This is a very common symptom of overstabilizd pools, btw.

As far as a more in depth discussion of the chemistry involved, the place to have that would be in the 'china shop' section of the forum since it really is beyond the scope of what the forum Administrator (Pooldoc) wants discussed in the general areas of the forum. You might want to review some of the info in the forum (and possibly some of the info over at Troublefreepool.com) about the chlorine/cya relationship and it's effects on pool chemistry. A good place to start is Pooldoc's sticky on his 'best guess chart' on chlorine levels for different levels of CYA that was posted by Watermom, one of the mods here.
http://www.poolforum.com/pf2/showthread.php?t=365

You can also search here or at Troublefreepool for posts by chemgeek. He has posted quite a bit of the theorectical chemistry on the chlorine/cya relationship.