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My new 20G Long tank was nearing the end of it's "fishless" cycle last week. At that time the water test results were as follows:

pH 8.0
Ammonia 0
Nitrites 0
Nitrates 5.0 (or slightly higher)

I went out of town for 3 days and when I returned, I had a heavy layer of what appears to be a dark brown/green algae over every flat surface in the tank. I removed all the large rocks and fixtures for cleaning but have not yet put them back in. The tank light is on a timer so the tank received just under 12 hours of light per day (as do my other 2 tanks). I repeated all of my usual test and got the following results:

pH 8.4 (possibly as high as 8.8, color scale is open to interpretation)
Ammonia 0
Nitrites 0
Nitrates 5.0 (just barely registering)

I repeated the pH test 3 times and got the same result each time. Could a heavy, sudden algae bloom cause the pH to change? I don't really think so but this was the only "wildcard" in the equation.

So you had a jump in pH and an outbreak of diatomaceous algae. Diatomaceous algae is a symptom of a new tank. Read about it here:
Skeptical Aquarist

Diatom control. Low light levels don't discourage diatoms. They can photosynthesize at dim light intensities even algae can't use. Often diatoms become less of an issue as an aquarium matures. It may be that higher light levels directly inhibit diatoms, or it may be that stabilized conditions ordinarily favor green algae, which overgrow the diatoms, as long as they get enough light to prosper.

Reducing the concentration of dissolved silica is an approach that's often talked about: tap water may be rich in dissolved silica. A Russian study suggested that high ratios of silica to phosphorus (as phosphate) encouraged the diatoms in the algae/cyanobacteria/diatom community, but that lower ratios of silica to phosphorus found planktonic green algae displacing diatoms. If such studies of "green water" algae grown in an illuminated lab flask are relevant to aquarium experiences (why would they not be?), they suggest that as phosphates build up in a maturing system, and as initial dissolved silica is scavenged by diatoms, the changing ratio Si encourages green algae to displace diatoms.

The phosphate-adsorbing pillows of aluminum oxide you can put in your filter will also lower silica levels in aquarium water; sometimes they have been represented as an option in diatom control. But according to biologist Lynn Margulis, diatoms are so competent at removing silica from the water to form their lacy tests that they can reduce the silica concentration to 1 ppm, which is below the value that a hobbyist's chemical testing technique can even detect. So diatoms can never effectively be reduced in number by trying to control silica in the water, though you'll often hear this attempt recommended.

Besides, as Craig Bingaman in an article on marine diatoms for Aquarium Frontiers, Feb 2000, said, "It takes more than silicate to grow diatoms, and if diatoms are growing, they are growing by using nutrients that might otherwise fuel the growth of other types of algae."

Instead, try more intense lighting, though not a longer photoperiod. Otocinclus, the miniature Loricariid catfish, are enthusiastic diatom eaters. Try them. More industrious glass-cleaning on your part may help. And patience.

Your pH may have been affected by the water in your aquarium "de-gassing". Essentially, water that is delivered from your water utility arrives under pressure with gas dissolved in the water. As this gas, generally, CO2 leaves the water as the water is heated and agitated, the pH of the water rises...

Just a guess, hope it helps.



__________
"To steal ideas from one person is plagiarism; to steal from many is research."
researched from Steven Wright

I was just thrown by the fact that the algae took all this time without being seen and then was suddenly everywhere. The pH jump was odd, too.