For the past several years of pool ownership, I’ve always had off-and-on problems with cloudy water. I’m generally pretty good at keeping up with the water chemistry, so I’ve always been a bit curious as to why the water clouds up so regularly. The pattern is the same every year: it starts out crystal clear, then after a month or so, the water slowly starts getting hazy.

The only way to get to the bottom of this is to apply the scientific method: assume that the problem is caused by x, try a known solution for x, and see if it works. I’ve worked at this over the past few seasons, and I’ve come up with three potential causes.

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Problem: Yellow algae
Cause: Lack of superchlorination

Yellow (or brown) algae presents as a fine “dirt-like” substance that accumulates on surfaces. When brushed, it dissipates easily and clouds up the water. It re-settles when the pump is off. I had big problems with yellow algae last year and the year before. At the time, I was superchlorinating very infrequently (only one or two times a season). This year, I have been superchlorinating weekly and also using a polyquat type algaecide semi-regularly, and I have not had an algae problem (yet). If this is the ticket to keeping it at bay, then I need to figure out the ideal frequency of superchlorination that will prevent algae blooms without wasting too much chlorine.

Problem: High pH
Cause: Prolonged use of hypochlorite sanitizers without adding acid to compensate

High pH and/or Alkalinity can cause cloudy water. Once this year I let the pH drift to almost 8, and the water was noticeably turbid. Adding acid cleared it up after 12 hours or so. I’ve found that supplementing the hypochlorite with a trichlor floater (in moderation, to avoid high levels of cyanuric acid) can help to keep the pH down, particularly during the hot months when the chlorine demand is high.

Problem: Inadequate filtration
Cause: Undersized pump and/or not running pump long enough

I’ll freely admit to running the pump as infrequently as I can get away with it, to try to save electricity. Unfortunately it appears that I’m paying the price for this in the form of cloudy water. Currently, the pump runs around 9 hours a day (6 hours in daylight and 3 hours after dark). With turbid water, a pH of 7.4 and no visible algae, I ran the pump for 24 hours straight and there was a marked improvement in clarity. So it appears that I need more circulation. This seems odd to me, because 9 hours really should be enough to fully turn the water over and keep it from clouding up. So I’m curious if my pump and/or filtration system is undersized. When I get around to it, I’ll measure my flow rate and see what kind of numbers I’m getting. If they’re low, I may want to consider a larger pump and/or filter. Until then, I guess I’m stuck running the pump longer if I want clear water.

We have a solar cover for our pool. It’s essentially a big, plastic sheet of bubble-wrap that sits on the pool surface. To remove it, we crank it onto a big reel — it’s pretty much a necessity to have a reel for these things, particularly with a large pool like ours.

After 5 seasons of dealing with solar covers, I’ve learned quite a bit about them. Contrary to what one might initially think, they don’t increase heating during the day when the pool is exposed to sun. In fact, they actually reduce daytime heating by blocking sunlight. There’s a bit of a greenhouse effect that heats the top foot or so of water, but the rest of the water sees less sun so doesn’t heat up as much. One desirable side effect of this is that it reduces chlorine usage during the day. So.. if you’re going on vacation and want to cut down on chlorine demand, put the cover on.

The biggest benefit of a solar cover is that it reduces heat loss and evaporation at night. So for the warmest water, you want to cover the pool in the evening and uncover it during the day.

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The drawbacks of solar covers are subtle and a bit paradoxical (hence the title of this post). The biggie: You’d think that using a solar blanket to cover a pool would keep the pool cleaner, but it doesn’t. In fact, you could argue that it actually makes the pool dirtier. It’s true that when the pool is covered, stuff will fall on the cover instead of into the water. And sure, you can even use a leaf rake to clear the larger debris off the cover before you remove the cover. But when it rains, the dirty and untreated rainwater will collect around the folds and creases of the cover, and then when you remove the cover, the dirty water (along with whatever debris is still there) all falls into the pool and fouls the clean water. Then, because most of the debris is already waterlogged, it immediately sinks down to the bottom of the pool. Contrast this to when the pool is uncovered: the skimmers collect all the debris before it sinks, and the rainwater immediately mixes with the treated water rather than puddling up on the cover.

Solar covers (like any cover) also encourage neglect of the pool. When the pool is covered, I find myself more likely to skip chores like adding chemicals, brushing, emptying skimmer baskets, running the automatic cleaner, etc. When you combine this with rain and extra organic debris, it can lead to an algae bloom pretty quickly.

Conclusions: Unlike conventional covers, these things really are not meant to keep stuff out of the pool. Use them only for their thermal properties (chilly nights etc), and keep them off during the day and in particular, when it rains. If you must use the cover when it’s raining to conserve heat, uncover the pool as soon as possible after the rain stops.

I’ve often thought about getting a leaf net cover to keep out debris. I think it would complement the solar cover nicely, and in certain instances the two could be used at the same time. I hesitate because I’m not sure how much trouble it would be to put on and take off, and how I would anchor it down to keep it from blowing away, falling in the pool, etc. When you think about it, another tradeoff with covers in general is that they are an impediment to getting in the pool. It’s much easier to go swimming when the pool is uncovered and ready to go. No one wants to spend time fumbling with a big, unwieldy cover beforehand. Still, in the late summer and early fall, when we’re swimming less frequently anyhow, the leaf net may be worth the hassle.