Fiber Clear in cartridge filter

I've read several places about using Fiber Clear (DE alternative) in cartridge filters to reduce the micron rating and 'polish' the water, so thought I'd try it out this year. I bought a 9 oz. bag and started dosing the skimmer a little at a time, and kept checking the pressure for the 1 psi rise, but it never rose (verified working pressure gauge). After dumping nearly the whole bag in and getting no pressure rise, I finally looked at the pool water and noticed it was getting cloudy. Whoops. So since my cartridges were 5 years old now, I bought some new ones and started over. Once again I fed a whole 9 oz. bag in (this time the water stayed clear), still no detectable pressure change. Since my filter is 420 sq. ft. (my research here and other places tells me that's way overkill for my 10,000 gal. pool, but I'm not changing it), I'm thinking it might take a lot of Fiber Clear to bring the pressure up. Would you agree?

I went through something similar in my pool this year as I thought I'd try out Fiber Clear before I was going to be cleaning the filters at the start of this season (I only clean them once a year since they are oversized 340 square feet for a 16,000 gallon pool). I added 2 9-ounce bags with no detectable pressure rise and no change in water clarity after a week or two. It didn't cloud up as your pool did. I E-mailed the manufacturer about it, but what they wrote back didn't make sense saying that it coats the outside area as in the circumference and not getting into the pleats. When I mixed up the Fiber Clear in the skimmer it's pretty clear that the particles are very small so would most definitely get into the pleats so would likely take a LOT of Fiber Clear to coat the filter.

Anyway, I ended up cleaning the cartridges and I don't plan on using Fiber Clear during the swim season. The water is in great shape and this was just an experiment. I may try again at the start of next year, this time buying more bags or maybe I'll just skip this entirely. It's not necessary, but still a bit disappointing that it's not more obvious how much is needed for a given size cartridge filter.

And yes, your filter is oversized for your pool, but that just means you don't need to clean it very often which for me is a great convenience.

Wow, I knew the filter was oversized, but didn't know how much. Your ratio is 47 gallons/sq. ft., whereas mine is 24/sq. ft., and you're saying yours is oversized still! Any rule of thumb on cartridge filters for sizing? Anyway, I got a mailer from Leslie's for a discount on 3# of Fiber Clear, so maybe I'll further the experiment and let you know.

Many filter manufacturers for residential pools go by a filtering capacity at maximum flow rate for one turnover in 6 hours. For my filter, the max. flow rate is 127 GPM so that works out to 127*60*6 = 45,720 gallons as their maximum pool size to support, but that's not particularly realistic. The filter area for a cartridge filter just means how long you can go between filter cleanings. With more filter area, you can go longer. I'm able to clean mine just once a year, but I have a pool cover that keeps the pool pretty clean most of the time. If I didn't have the cover, I'd probably have to clean more often. My filter is probably oversized about double, but again there's no exact magic number here. Clearly, yours is even more oversized. That is not a bad thing -- you probably don't have to clean your filter very often.

The other extreme is getting a small cartridge filter with one cartridge that you would need to clean more frequently, possibly monthly. Some people find that easier to do. There is no question that cleaning my 4-cartridge system is a PITA, but only having to do it once a year isn't so bad.

Well, the experiment is over for now. Over the course of a couple days I put in a 3# bag, plus the first 9 oz. bag, and still no change in pressure. I have to conclude that the filter is way, way, way too big, and I'm not going to shell out so much money just for that perceived 'sparkle' and 'polish'. I've been using the pool for 5 years now without it, and I've somehow managed to enjoy it anyway. Thanks for your input chemgeek.