The peat/sand recipe is not foolproof. See my tear streaked visage? This is from having dealt with the Curse of the Green Snot for so long. I used the p/s recipe, but in aquaria where there is enough light to support flowering, there is also enough light to support algal growth.
The key, as far as I can determine, is to include companion species in the aquaria. Peat and sand is not enough. My aquatics are shaded by duckweed, and I have rushes growing in with them. These plants absorb whatever nutrients are in the aquaria for their own growth, leaving a good nutrient free home for the Utricularia. I have given up on transparent aquaria. They always grow algae. Now I use 1 plastic dishpan for each species. There is a layer of peat and sand on the bottom, along with some small pine branches and a few old leaves from the fall. It is filled with clean rainwater and kept half beneath some long thin leaves of a lilly, and the surface has a sheet of white plastic floating on it. The sun shines on this plastic in a broken manner for most of the day. The lilly leaves break up the sun, and the plants grow quickly and have been flowering this season. I grow macrorhiza, intermedia, radiata, gibba, and purpurea in this way.
Well, its not too esthetic, but the plants seem to thrive! Most freeze solid for the entire winter, others are close to freezing from Nov.-Mar. When they start to put up scapes in the early summer, I remove the plastic sheets, and use reversed "flats" to provide additional breaking of the sunlight. In nature the plants are often protected by sheathes of.....algae! This screens out the UV. Once the algae is discouraged, the plants are far more likely to sunburn. This is why a screen is needed of white plastic.
The algae comes from the rainwater. Using distilled water is no answer either, since the plants need prey. I suppose there is something sterile in the petstores that would serve, but my resources are limited, so I add some pond water, which is a guarentee that algae is also being added. I conclude that algae in with the aquatics can't be prevented, only dealt with and controlled.
I had purpurea in a transparent apothocary jar for the better part of a year. Then, when the temps became warmer and the light levels more intense, the algae appeared. In nature this occurs as the season progresses from spring to summer. Plants sprout from turions early in the season when water temps are still too cold to support algal growth, and the light levels still mild enough to allow growth without sunburning the plant. The growth at this time is very rapid. As the warmer weather arrives and the light levels are more intense, the algae begins to sheathe the plant, and the duckweed becomes associated with the whole complex, rooting in the algae. It is a very natural process which the Utricularia need to protect themselves, and also explains the difficulty in trying to control what Nature wants to be.