A drop in pH, and this happens in a matter of days, will make those compounds water soluble and therefore available to the algae spores. Please note, that the pH levels slightly vary within the tank, and a drop in one spot can have an effect . Silicate The most common points of entry are the substrate, salt, water and dying diatom algae. Another form of silicates is silicid acid, created by decaying organic matter. Same as phosphates, they can be water insoluble at a high pH, and become readily available with decreasing pH levels.
Chlorine is less stable then chloramine and airs out in just a few days.
Some hobbyists simply let the water age for a couple of days before doing the water change, thus airing out the chlorine. Chloramine is much more stable. That is why communities switch from chlorine. As it is very stable, it does not air out even if heavily aerated. In the blood, just like nitrite, it destroys the oxygen carrying cells.