Don't Blame Semi-Hydroponics!
Sorry for not responding sooner. Being at the Outer Banks on vacation is keeping me way too lazy. (Not to mention that the fish are biting better than they have for the last several years.)
Let me start by saying that I do not consider "my way" of S/H culture to be perfect, or the only way it will work. What DOES bother me though, is the way folks do stuff without necessarily realizing how they are affecting their outcome, run into problems, then credit the general concept with the problem. It is illogical and flies in the face of scientific reason, which clashes with my science/engineering background.....maybe I'm just picking up the flag from Bert Pressman.
1) Dump the charcoal that's in the medium and do not reuse it. Charcoal traps (permanently, unless you have the right heat treating equipment with atmosphere control) all sorts of stuff that can be toxic to your plants, whether that is directly, or by affecting the pH or other chemical aspects of the root zone environment.
2) The reason for filling the S/H pot to the top and letting it drain is not just to rewet the medium, but also to re-dissolve minerals and flush the pot, especially the reservoir. Between changes in the nutrient content and the changes created by the gas exchange at the root system, the chemistry of the solution in the reservoir can be totally different than what you put into it after only a few days.
3) Likewise, pathogens are not the only reason to avoid sharing a reservoir (via a tray) among several plants. The solution chemical effects I mentioned above can vary from plant-to-plant. Sharing the reservoir "forces" the chemical changes on the other plants.
4) Reducing the volume of the reservoir so it will dry out faster defeats the whole purpose of S/H, and in fact, isn't semi-hydroponics at all. It is simply "traditional" culture using the inert LECA medium. Roots must grow cells that are accustomed to the contantly-moist environment in order to take advantage of it and thrive. Letting the medium dry out in between waterings does not allow that to take place.
Also, while algae is not really detrimental, unless you let it go to the extent that it clogs air flow, the best approach is prevention. A monthly watering with Physan (1 tsp/gal) or bleach (1 oz/gal) will do the trick.