Comparison of substrate types for germinating cactus seeds - which works best?

I'm never sure what sort of substrate should be used for germinating cacti. So here I am starting an experiment to run over the next few months. I have sown 3 types of seeds - Astrophytum Capricorne, Gymnocalycium Damsii and Echinocactus Grusonii - each into 2 separate substrates, one a standard bought potting compost, and one a home-made mix of 50% coconut fibre, 25% river sand and 25% perlite. The former will have a good balance of nutrients in it, suitable for potting up adult plants. The latter will have very low nutrient content, but cactus seeds should be able to start growing without too much nutrition. I've found that a nutrient-rich substrate leads to quite a lot of moss growing under the grow lights. This moss is not necessarily a problem but it can rather swamp the small seedlings in the early stages. I aim to avoid this problem altogether with the home made subsbtrate, and it will be interesting to see how the two compare.

I'm sorry everyone if you were looking for an answer to the question "what substrate should I use to germinate cactus seedlings" then you will have to wait a few months until I have some results! I will provide an update when I open the bags, and may provide an update before then if there is anything interesting to show.

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Happy cactusing everyone! :-)