Molch is right about the iron getting accessable for algae. And the problem with the sand is, i believe, caused by the aggrevation of water movements through the substrate. Though sand should work well in an established planted tank, i don't think it does good in a tank like this, where there are not many plants and there are a lot of rotting processes in the soil without the oxygen supplied by plant roots.
Temperature in my tank ranged so far (i set it up again in June i think?) from 16 to 30 degrees, recently around 18'C (64F), i don't think it will drop below 14 in coldest days. The light is 30W compact bulb and one 8W T5 fluorescent tube.
Plants are Cryptocoryne wendtii, Hygrophila siamensis, Hygrophila polysperma, Anubias barteri, Glossostigma elatinoides, Hedyotis salzmannii, Limnophila sessiflora, Myriophyllum spicatum, Najas guadelupensis, Echinodorus sp, Rotala rotundifolia, Echinodorus barthii, Cabomba caroliniana, Bacopa amplexicaulis, Ceratophyllum demersum. I don't measure the water parametres.
I do very small water changes once for a month.