Btw when you do water change careful do not put too much water that if return pump of off, you system flood.
Best way to do that is: turn off your return pump and let water drain to the sump.
Then siphon all water out of the sump. Clean it. Then fill in the sump with the new water until all sump is full to the top minus half an inch or so. Then turn on your return pump. This way if return pump failed and stopped working, your sump will not fill up and over flow. It will fill up to the point you originally filled it. Once you do that adjust your ATO floats to the new operational water level.
So basically this firstbwater change you will not be able to vacume the sand or take water out from the DT. Which is ok. Next time you can do that.
Unless you can precisely estimate how much water you can pull put of the system them push in without overflowing your sump.
If later we wanted to increase the water level in the sump more, I can walk you through how you do that( it will be a matter off your return and overflow pipe height inside the display tank that decide how drain water go in to the sump when return pump is off)...I hope am not confusing you here..kind of hard to explain in txts...