I've heard varing rules of thumb, but some where between 3 and 5 gallons per a plant is good. And I think that means 3 to 5 gallons in reservoir size, not the total amount of water you have in there. I am in the middle of my first grow and I am using a 4 plant DIY in a 14 gallon tub. I have only been keeping 6 gallons in the reservoir though. If you check out the link in my sig to my journal, you'll see that the plants seem to be doing well. For my next grow I would prefer to use a slightly larger rubbermaid if only to have more surface space, maybe a flatter 18-20 gallon tub. So I think you might be happier with the 72L tub, 30L would be awefully tight for four plants I think.