that really depends on the tank and what i am planting. i am a huge fan of cryptocoryne species, and they are HEAVY root feeders, so any tank that is going to have crypts in it, always has soil as it's base, with a (normally) black substrate capping it. i have used all the name brands, aquasoil, flourite, floramax, ecocomplete, ect. they are all pretty much equal. right now, i am running a crypt/java fern/anubias tank that has a soil/kitty litter base. if i have no root feeders, i will run fluval plant stratum or fluval shrimp stratum.

i use soil whenever there are root feeders, mainly because i HATE root tabs for fertilization. why spend $20 every other month for fertilizer tabs, when soil will do it for 2 years easy? and i have pretty much switched over to this soil exclusively, for both terrestrials AND aquatics:



it has all the goodies a young growing plant needs. worm castings, bat guano, elp, fish bone meal, soy bean meal, and more. here is what my "soiless" soil does: this is a pic of a plant that i have grown in several different tanks, under different conditions. i have grown this in an aquatic state, no growth, and emersed state, the same, no growth. i planted in my 125 palu, and there is dramatic difference. can you tell when the roots hit the soil? remember, there is about an inch and a half of coco fiber covering the soil. but i think the result is dramtic:

that plant has been the same size for almost a year, and now, obviously, it is growing well. funny thing is, neither i, nor the guy who i got it from, knows what the plant is. LOL it was in a clump of plants he gave me. he didn't even know he had it in his tank....lol