I agree with rayjay that water gets the brunt of the blame often for algae, but just because your water is pristine doesn't mean it's going to stay that way forever. I'll just pass on a little story I've lived the last couple months.
I live near Seattle, WA where we have exceptionally great tap water. My city water is treated with chlorine. Last summer, when planning what to do about water for the tank, I tested and retested my tap water. Except for the chlorine, it was fantastic - no nitrates, no ammonia, no phosphates, no nuthin. So I go out and buy a Kold-Steril filtration unit as I don't really like the idea of wasting a bunch of water, and I like the idea of water "on demand." (For those that don't know what a Kold-Steril unit does, it's not RO
... but a series of polyfilters in various porosities, coupled with a carbon block, that will take out heavy metals and chlorine.)
I install the Kold Steril and everything's great for several months. In my 10g quarantine tank, I start to realize that I just can't get my nitrates down below 10ppm. Even with massive water changes, I still can't get it below 10ppm. I check my stored saltwater and find IT has 10ppm nitrates! I check my tap water out of the Kold-Steril... yup, 10ppm nitrates. OK... it's not the end of the world, but I really don't want my "perfect" water change water to start me out at 10ppm of nitrates.
I scrambled to figure out what to do, and decided to bite the bullet and just hook up a double canister DI
unit to the output of my Kold Steril, and restrict my flow with an industrial flow meter. (Hats off to the folks at thefilterguys.biz for all their help!) I ended up with pure water, running at about 40 gallons/hour.
Eventually, I got a hold of the water company to find out why the nitrates were so high. Federal standards limit them to 10ppm. They said their yearly testing showed only 1.5ppm nitrates in my water, but they'd send someone out to draw a sample and test. I have to hand it to them... they got on it pretty quick and came out to my house and got a sample, as well as a sample from the storage reservoir near our house. After a couple days, they got back the lab results and it was 1.5ppm nitrates. I went home and tested with my API test kit, and ended up with 10ppm nitrates yet again. When I tested my pure DI
water, as expected, I got 0ppm nitrates.
Sooo... what's going on? I'm not sure. I've asked API via email for an explanation and haven't heard boo from them. I'm guessing there's something in my water that is triggering a false reading for nitrates because I trust the water department's lab results way more than I trust my little "compare the colors" test.
I guess my long-winded point is that I thought I had pristine water. And I did... in August of 2006. But come November 2006, something is different and testing shows it as nitrates, even though I'm sure it isn't. Rather than scratch my head and wonder, I'm playing it safe and going with water that I KNOW is pure.