It didn’t make sense: I had enough chlorine in the Kelly pool to make stingy eyes, but I was still getting algae on the steps. So, I called in Mark Larimer of Save Water Mobile Pool Filtration in Lakeside (619-328-9099; savewatersd.com).

“If you use chlorine tabs in your pool,” explained Larimer, “you’re continually adding more conditioner [cyneric acid]. It helps keep the chlorine from burning off. But once the conditioner gets over 100 parts per million, it starts to act as an umbrella for the chlorine molecule, and it gets hard to keep the pool clean. Even with a high chlorine level, you can still get algae because the conditioner won’t let it do its thing. And you can’t get rid of conditioner unless you either drain the pool or use reverse osmosis.” Larimer provides the latter.

“It’s cheaper to drain a pool, fill it up with tap water, and then go down to the pool store and buy $50 worth of chemicals to throw in it,” granted Larimer. “Factoring in the cost of water, you’ll be up and running for under $300. But the $200 more you pay for filtering is cheap insurance against any problems with your plaster. When you drain a pool, you’re removing 200,000 to 300,000 pounds of water weight from the pool. The exposed plaster can blister or get eggshell cracks. There have even been cases where pools pop out of the ground, though that’s more likely to happen somewhere like Santee, where the soil has a lot of expansive clay. Replastering a pool can cost from $5000 to $10,000. Though some people do drain their pools and have no problems — it’s just a gamble.”

Larimer explained his process. “Before doing a job, I do a water analysis. If you don’t need my services, I’ll let you know. Your total dissolved solids may be high, but you may just need to upgrade to a better pool filter. I see lots of pools where the filter is undersized or just too old. The equipment I use is almost like a dialysis machine for the pool. We’re stripping out the total dissolved solids: the calcium, the minerals, and the conditioner levels that have built up over time. The water level of the pool doesn’t really change. We have a hose that’s pulling water out for us to filter, then another hose putting it back in. We pull 40 gallons a minute. Of that, 10 gallons are being dumped down the drain — full of conditioner and microbacteria and calcium — and 10 gallons are being added as make-up. Depending on your [dissolved solids] levels, you’ll lose 15 to 30 percent of your original water. The whole process takes eight to ten hours.”

Larimer recommends filtrations every two to five years for optimal pool longevity. He charges $575 for filtration. “That includes a chemical reset when we’re finished, making sure all the levels are correct — even salt, if it is a salt pool.” And if you call for an appointment during the winter season, it’s only $475.

Bruce Wettstein, co-owner of Pool Service Technologies in Escondido (800-535-0778; poolservicetech.com), pointed to another advantage of filtration instead of draining. “You do save water, since you keep about 85 percent of your original water. But city water is going to have minerals in it, predominately calcium. That’s what people worry about because it leaves a white line around the swimming pool.”

Edit 11/1/18: Pool Services Technologies no longer deal with pool water purification services.

Like Larimar, Wettstein uses reverse osmosis. “Once we pull the water out, it goes through a prefilter, a sand filter, and a UV light. Then it hits the reverse osmosis membrane, where pretty much everything is removed. Even the make-up water we’re putting in gets filtered. Nothing coming out of our rig contains any hardness or minerals.” Of course, that means if you have a salt pool, “the salt is getting filtered out and you’ll need to add salt back into the pool.”

Pool Service Technologies charges $475 for pools up to 20,000 gallons, $575 for pools up to 25,000 gallons, and $675 for pools up to 30,000 gallons. “And we always run a seven-point water analysis both before and after the service. First, we make sure you need the service, and last, we let you know what you should do to get the pool chemicals up to their optimum levels.”