You won't be able to (easily) retrofit a solar collector to existing aquarium equipment.
AC is integral to the design of the motors and lights and they will not operate on the DC output of a solar panel. To do so you'd need a LOT of solar panels to make enough power, a battery bank (sun doesn't shine at night) and an inverter (to convert DC to AC). It would cost hundreds of dollars just to save pennies on electricity. Not worth it.
Now there are some items that might be cost effective to replace.
LED lamps operate on DC and are super efficient. It might even lend a neat effect as the lights brighten and dim as the sun moves across the sky.
The most cost effective option would be to make a solar powered heater. It's not hard to convert heat into...heat. You'd need a thermostat, a pump (any pump, you'll power it off mains), and a bit of piping to make the collector. not too bad. I bet you could save $100 in electricity every year if you did it right.
I'd keep the standard heater in there too, just in case it's a dark day.