Diatom control

Diatom control. Diatoms are everywhere in freshwater and in the aquarium. 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 outcompete and even physically  overgrow the diatoms, as long as they get enough light to prosper.
 
If you adjust your pH, or reduce the dissolved organics in your water, you may temporarily knock back the diatom population, but other species of diatoms that are more suited to the new conditions are quite likely to replace them.
 
What alternative strategies do you have?
 
Reducing silica. Reducing the concentration of dissolved silica is an approach that's often talked about: tap water may be rich in dissolved silica.
 
Reducing phosphate. 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.
 
The changing ratio of silica to phosphorus. Mark Berzezinski's 1985 study of marine diatoms 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:P encourages green algae to displace diatoms.
 
Intensifying lighting. As Craig Bingaman said in an article on marine diatoms for Aquarium Frontiers, Feb 2000, "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. 
 
Diatom-eater. Otocinclus, the miniature Loricariid catfish, are enthusiastic diatom eaters. Try them. More industrious glass-cleaning on your part may help. And patience.
 
Yellowing green water. If your "green water" develops a yellowish tinge, it's unlikely to be caused by a bloom of free-swimming diatoms or, even less likely, by dinoflagellates. It's more likely that many green algal cells are dying. Do an emergency water change immediately, to dilute the toxins they are exuding.