I change it by using a gravel syphon hooked to a garden hose (the garden hose goes outside onto the garden) syphon out about 25%. Then refill by bucket (i cant use the hose as it only connects to town water, i use rain water from a watertank to supply aquariums)

I currently also have running

2x 1ft tanks
3x 2ft tanks
2 AquaStart tanks

I have 3 tanks not currently running (1x 3ft is close to being filled)