Only perlite and vermiculite? I'd plan on using fertiliser every time you water, starting now. Very dilute though, less than 1/4 strength. I can't claim to have mastered this essentially hydroponic technique yet, so maybe you should ask in the hydroponics section and then go with 1/4 of what they recommend :)

One 13W fluorescent is insufficient light. Cactus seedlings don't need extremely intense light (full sun is too strong) but you should be aiming for 1,000 - 1,500 lumens. That's about what you will get 2"-6" from a simple fluorescent light fitting. Plan on 20W per square foot of illuminated area. Compared to the light coming through the window, your fluorescent is probably just a waste of electricity. Unless you need a reading light at just that spot :) Watch out for seedlings that go strongly red or pink, they may be getting too much sun, or even just too much indirect light.

Isolated red spots could be fungal. A cactus that loses its growing point before it develops multiple areoles is dead. Once it has multiple areoles, it can resprout if the main growing tip is killed, assuming the whole thing doesn't rot. Or they may indicate an insect problem.

I wouldn't worry about the upside down ones, they will sort themselves out.

I don't see why this would work like hydroponic growing. In hydroponic you have floting water in the bottom. I only have the water stored in the vermeculite (and possible a little in the perlite). Why would the plants use less nutrients then when planted in soil? I can see that the soil have some nutrients to, but in the long term it would make no difference, or?

Using plastic bags also reduce the evaporation so this would reduce the concentration of the nutrients. On the other hand, a too low dose is generally safer then a too high so I try with just a little. They are still growing fine. What is the first sign of low level of nutrients? I guess it will be different for different nutrients.

/David

Be careful which fertilizer you use - the "blue" soluble types, like Peter's or Miracle-Gro, will have a strong tendency to accumulate salts in the soil that can only be alleviated with frequent drenching, which is not advisable in a cactus seedling mix...I would stick with a something like Schultz soluble cactus solution at about 1/4 to 1/2 strength, in my experience that particular brand does not cause salt and calcium leaching.

I once had it so bad that the seedlings and soil surface started developing a calcified crust that smothered the plants. I learned my lesson.

I use a cactus solution named something like "Interflora Kaktusnäring". It is probably not avalible outside of Sweden:). Content: 4% Nitrogen, 3% Phosphorus, 7% Potassium. It also contain traces of magnesium, iron, copper, boron, manganese, zinc, cobalt and molybdenum.

/David