thanks so much guys !! i'll try to paint it =)
I've used several options to coat the back of tanks in black, including the usual black window film, Plastidip rubberised spray paint and Krylon black spray paint.
Black window film:
Depending on the brand it can sometime be difficult to apply and get the bubbles out. In my experience longer term the edges get lifted during cleaning the outside of the glass and other regular maintenance causing bubbles. Some bubbles eventually seem to form in more central areas too. The film is okay for short term, but not for long term in my opinion. It is easy to remove though. Don't ever be tempted to use Jewel Poster Fix to attach the film - absolutely horrific stuff, that continually leaks its oily nastiness down the back of the tank, and takes half a bottle of washing up liquid to remove!
Plastidip:
Applied via a standard rattle can. It goes on easily but doesn't cover overly well, you need to position the tank to you can look through the panel you're painting and get someone to shine a torch through from the other side. It took about one and half cans to cover a 600 x 400 pane. It looks like you are putting far too much of the stuff on, but you need to, to cover properly.
Masking off is problematic - you need to mask all areas around the panel off well to prevent overspray, but it is important you remove the masking whilst the coating is still fairly wet. The first one I did, I left it to mostly dry before removing the masking, and the masking just pulled loads of the coating off the glass at the edges, so I had to re-mask, and respray those areas to infill. On the second one I let the coating just skin over, and then removed the masking which worked fairly well.
Plastidip dries to a rather texturally satisfying matt rubber finish that appears to be fairly durable so far. I haven't tried removing it, but I imagine it will come off without too much effort.
Krylon black spray paint:
I've used Krylon spray paint for numerous things in my tanks, from darkening red lava rock with the brown variant, to painting white and clear pipework black. It is a paint that goes on very easily and dries very quickly to a good quality smooth finish.
Spraying it on glass was a little trickier though, mainly because you need to get a lot of coverage to completely block light. Because I'm generally pushed for time (read: "impatient") I rushed it and sprayed the paint too heavily. This resulted in there still being solvent in the paint on the glass, which eventually off-gassed leaving loads of little bubbles which let light through. So I then had to rub it gently to break all the bubbles, and then respray. What I should have done is laid down numerous thin coats, and waited for each to dry before applying the next.
Unlike the Plastidip, the masking can be left in place until the paint had dried without issue. The paint finish is likely the most durable, but will likely also be the most difficult to remove (should I ever choose to) - presumably requiring a craft knife blade to scrape it off.
In all cases it's always worth cleaning the glass with an alcohol based cleaner, and lint free cloth, before painting.
In the future I may try and find a brush or roller applied paint rather than a spray, for less mess. Bear in mind on glass I suspect you'll need to use a solvent based paint, as I don't think water based paints (such as some of those suggested above) will take, and may just bead up.