I really think you should turn your old tank into a frag tank Mike (@i-CONICA)
Is that because my old tank is in your front room?
I've got that little 125L, and I have considered it, but to be honest, I don't see the point, it's not as if I've got no space in my main tank, and that's by far the best coral growing place I could manage. There's £800 worth of lighting above it, I'm not going to replicate that over a small 125...
I already maintain high reef levels in my main display tank, so corals will grow quicker in there than I could ever hope for them to do in a frag tank.
I do keep getting the urge though, just for something else to do...
A frag tank doesn't need much, a skimmer, heater, powerheads and light, that's it.
Corals don't produce much waste, so the surface of the tank and the nugget of rock the coral is on is enough surface area for biological filtration, so you don't need any live rock or other filtration...
The point of a frag tank though is that it can be ugly, no need for a sump, can use a hang on skimmer, and no real need to feed at all or very much, so nutrients remain very low. An auto-top up unit is really important though, as typically frag tanks are wide, flat tanks with intense lighting that'll evaporate a lot, and corals typically need stability more than most things.
Here's an excellent video from that long haired guy at Bulk Reef Supply.
He's awesome, does really good videos, all spot on advice, totally impartial.