Plain sand probably isn't the best substrate for your cube.
If i where you i would start over and add some sort of mineral in your substrate. In my first aqucube (see below) i used pea gravel and JBL aquabsis underneath with pressurized Co2 and Nutrafin PlantGrow. The second is with ADA substrate and ADA brighty step1 and pressurized Co2.
The lighting needs to be upgraded. Go to B&Q and find a halogen with a reflector built around it at 20W or 10W which ever you prefure.
Have a look in my journals , links below.
As seen in the latest issue of Practical Fish Keeping.
The latest design.
My first Journal for the cube SHAKKIE ( top pic )
http
/www.fishforums.net/index.php?showto...&hl=Shakkie
The second journal for cube design Soar. ( bottom pic )
http
/www.fishforums.net/index.php?showto...&hl=Shakkie
Any question, dont be scared to ask.
Regards,
Graeme.