Hello; I was going to suggest a project on the cycling process but another clever member beat me to it.
How about somethin on the physical aparatus that operates a tank setup? How a heater thermostat works? (bimetal springs, watt per gallon, convection demo with heat source)
diurnal light cycles? Adaptations of plants for aquatic living( submergent, emergent)? UV light?
Life cycle of parasites common to aquaria? (ick)
Good luck