I have just re-built the lid on my 70 gal tank and I think I've found a great material to use as a reflector for my 4 x 40 watt tubes. It's material designed for use behind central heating radiators. It comes in a large roll about 20" wide, it has a plasticised metallic reflector on one side and thin insulating foam on the other.
I glued it directly onto the lid using spray carpet glue and it seems to work really well
The only downside is that it cost about a tenner from Homebase
, but there is enough left to do another couple of tanks
I glued it directly onto the lid using spray carpet glue and it seems to work really well
The only downside is that it cost about a tenner from Homebase
, but there is enough left to do another couple of tanks 
