Newbie flush

Hey, first time growing here, reading a lot trying to learn etc etc. I hear about flushing but my system I don't see how it could be done. I have a 10 gallon fish tank, the water is -unreplaceable- theres no res and if I were to take out all the water to flush, would it cause stress from changing waters completely?

Im not sure what system you are using. Plants are fine being withut water for a shirt time.

Its a 10 gallon fish tank, a foam board with a few holes to balance it and hold your cups. It floats in the water with a airstone, you can't take the plants out, I guess its a pretty bad design.