My Experience with Algae and a New Sand Filter

We have about an 18,000 gallon gunite pool, and had a very old Anthony DE Filter and a 2 HP motor. The motor sprung a leak and since it was very old and corroded we replaced it with a 2hp Pentair. During the interim our pool was not filtered for about 10 days - this was early April. By the time the new pump was put in, the pool was as green as a pond - really almost black. Our kids called it a "nature preserve" and ducks started to live in it. This occurred very quickly - we live in Texas and the temperature soarded to the high 90's during the time that the pump was out of commission. During the first day the new pump was running I noticed DE poring into the pool - I opened up the filter tank and sure enough the filter screens were all ripped up.To replace them (9 in all) would have cost anywhere from $150 to $300. I was afraid that even new screens might not last long given the new pump and the excessive algae.

So I decided to get a new filter - after perusing this forum and some other sites I decided on a sand filter - mainly because I had grown up with one and it lasted for at least 20 years with no significant problems and very little maintenance. I was sick of cleaning the DE filters and buying expensive replacement filter screens. One Pool store I went to tried HARD to sell a cartridge filter and said he hadn't installed a sand filter in 10 years. Another pool store was much more favorable toward sand and also mentioned zeobrite as an alternative to the filter media. (I can't understand the heavy sell on the cartridge and DE filters - somebody explain? It must be the stores rake in $$ on the cartridge replacements).

Anyway, I decided on the sand filter with the zeobrite filter media and had the anti-sand pool dealer install it (a 600 lb model pentair - he had the best price but I was still a little nerverous because of his attitude and I was afraid he wasn't up to speed on sand filters). The guy installed it okay although I had to tell him to use pea gravel on the bottom - he didn't know that even though it says to on the zeobrite web site and in the manufacturer's instructions.

Anyway, then the real work began. The pool was now REALLY black and ugly. The dealer INSISTED that we would have to drain the pool and pay him $500 to acid wash the sides. Well, the filter was installed on a Tuesday and I started to shock beginning Wednesday. I read this forum and started the bleach routine - shocked it about every 12 hours with 7 gallons of bleach (I was under the impression that we had a 30,000 gallon pool - LOL - until I did the measurements and figured it was only about 18k - so I could have done w/about 4.5 gallons to shock). Once or twice I did use shock stuff I got at the store but basically I used bleach (Clorox ultra). I also adjusted the PH by adding acid until it was within acceptable range. The pool stayed green. I started to vacuum manually and backwash frequently as the pressure increased. The vaccum seemed to pick up stuff - at one point the pressure increased almost immediately to backwash range after I started vaccuming. Also, all during this time I maintained about 2 3" "puck" trichloro-s-triazinetrione tablets in my skimmer - the usual maintenance dose. I know these are frowned upon but I have an 80 gallon bucket to get rid of and it seems like I needed some stabilizer. Since I do not have a CYA test, I was never able to test it and I don't know what the CYA level is. Maybe the high temperatures in Texas makes this less of a problem?

On Thursday night at 7 pm, I added 7 gallons bleach and 20 oz of 40% algaecide by "Pool Brand" I got from Sams Club (dimethyl benzyl ammonium chloride (I later read on these pages that Ben only recommends polyqot but this seemed to work for me). At midnight, Thursday I added another 20 oz of the 40% algecide but no more chlorine.

On Friday morning the pool was blue! But it was cloudy!! Over the next few days I then tried everything to get rid of the cloudiness knowing that it was the dead algae. From adding a cup of DE to vacuuming. But what finally worked - probably the following Wednesday - was adding some clarifier - "Super Water Clarifier" from Aqua Chem I got from Home Depot - 1 oz. treats 6,000 gallons - unfortunately I misread the label thinking it was 2 oz/6,000 gallons, which together with my miscalculation of my pool gallonage led me to over dose the pool - which the label says can do the OPPOSITE of clarifying, but fortunately in my case it didn't seem to matter. The next morning I could start to make out the bottom. I added more clarifier and kept filtering and after about another 3 days it became crystal clear (see pics that were taken I think on that Friday - about 10 days after starting shocking - the pics don't do justice I don't think maybe because it got even clearer later).

Okay, so here are my questions/comments:

1) Does zeobrite really work or not?? Does anyone have experience with it doing a better job than sand? It's supposed to have about a 0.3 micron filter ability but in my case it seemed not to work that well - I had to add the clarifier in order for the algae to clump together enough and get caught by the filter. Although, I will say the zeobrite did catch the algae but at such a slow rate - I calculated that it was filtering at a rate of 1 inch every 12 hours or so - meaning I could see one inch further into the pool every 12 hours. At that rate it would have taken a month or so to see the bottom! Once I added the clarifier it worked much faster.

2) I have to admit that a DE filter MAY have been better at getting out the algae quicker - it seems like my prior filter was quicker on getting out the algae but I probably would have had to do more than just backwashing as I did with the sand filter - I would have had to manually open it up and clean out the filters with that much algae. But with the sand filter, now that I know to add clarifier, I think I can live with the slower filtration rate.

3) Is bleach REALLY less expensive than shock?? I was spending about $14 for 7 gallons of bleach, as opposed to what would have been about $7 for about 3 of those 2lb shock packages, both doses assuming it was a 30,000 gallon pool (which it's not) and trying to get to 15 ppm. If bleach is less expensive then I assume that the 3 x 2 lb packages just don't add as much ppm chlorine? My goal was 15 ppm. By the way, I did all my testing with a retail "drop" test kit - I didn't have Ben's fancy kit. I'm trying to steal myself up to spending $80 for a test kit. Maybe some day...

4) Is trichloro-s-triazinetrione bad? I know tablets are frowned on because it can lead to overstabilization, but does trichloro-s-triazinetrione add CYA? It says on my bucket of pucks that they stabilize.

5) What are the black gritty spots on the walls of my pool in the one picture? I think it's the plaster coming off on the pool and the dirt/algae settling in? Or is it just algae? It doesn't rub off. Do I need to have it replastered?