[Adam98150]

Setting up a 70l and according to a substrate calc I'd need 20kg of substrate to get a depth of 3". Now there's no way I'm spending 75 quid for 3 bags of flourite. Would it be okay to use one bag of flourite black sand and then mix it with regular black sand? Basically it'd be one third flourite and two thirds regular sand.

 

[MCSLABS]

You are doing exactly what I did, use mostly sand and then some fluorite where I planned on planting.