Just got this tank for £12.50 from eBay 
(but silver)
I haven't seen a filter like it before though and not sure if it's working properly.
Water is drawn up and trickles onto the sponge which is in a plastic sort of cage in a tray. Beyond this in the tray are 4 plastic balls. The sponge is barely submerged and the balls aren't really at all. Is this right?
It has no heater, just pump and light which work from the same adaptor (so can't have a timer for light
) Next to where the pump plugs in is another hole, so I wonder if a heater could be run from the same adaptor?
Otherwise, it will have to be a standard heater, but not sure how it would fit?
The lid lifts off, so it wouldn't have to thread through the gap. Quite a bit of the wire would have to be submerged though for it to reach the back wall. Is this OK?
Thanks for any help with this

I haven't seen a filter like it before though and not sure if it's working properly.
Water is drawn up and trickles onto the sponge which is in a plastic sort of cage in a tray. Beyond this in the tray are 4 plastic balls. The sponge is barely submerged and the balls aren't really at all. Is this right?

It has no heater, just pump and light which work from the same adaptor (so can't have a timer for light
Otherwise, it will have to be a standard heater, but not sure how it would fit?

The lid lifts off, so it wouldn't have to thread through the gap. Quite a bit of the wire would have to be submerged though for it to reach the back wall. Is this OK?
Thanks for any help with this