You don't necessarily have to replace the solution every seven days. But do keep track of how long between changes.

here's the rule you should go by (dug out of latewood's recipe thread):

Do a full reservoir change. you have a 40 gal res? lets say you fill it to the top with 40 gallons just for the sake of ease. you'll no doubt be doing top offs along the way. Let's just say every 3 days you have to add 5 gallons of water. Well once you add a total of 40 gallons it's time to change the entire reservoir.

That's not the optimum situation. but it'll work. Just keep track of what week you are on and make sure you are giving them the right dosage of nutes as outlined in latewood's thread.

make sense?

cool.
Skeet