It is not harder to keep just need to be able to keep it more stable. Like Ntl555 said think of it this way, you have a glass of water and drop one drop of red food coloring into it. How much red is there in the water? Now drop that same drop in a pool and would you even be able to see it? At first but when you mix both you will have red water in the glass and not even a trace of it in the pool. Same with tanks. That is why everyone tries to get as large of a sump as possible under the tank to give more water volume. Hope that helps out. Not saying it cannot be done as many on here have kept the 3gal Picos and had a competition last year for about 6 months. It is just harder to maintain one.
