Choosing a storage tank that will fit your needs is very important. It often seems like a nice idea to have a bunch of small tanks but in the long run, it typically ends up being cheaper to have larger single tanks if it is practical. One of the big constrains is that you ideally want a 2000L or larger tank to be able to fit an In-Tank UV light. If you need to choose between two 2500L tanks and one 5000L tank keep in mind that for the two tanks you will need a UV light for each thus increasing your running costs over time. Another thing to look at is tank maintenance, it is important to have a way to drain the tank and in extreme cases be able to get in the tank to clean the inside of the tank.