i appologise in advance for not having any actual photo's of one because i havent had to make any tanks for a while now and kinda threw mine out but its sooo simple.
all you need is;
a pack of mdf screws
1 x full sheet of mdf or hdf, NOT chip or ply(around 22mm or above)
2 x strips of solid straight timber roughly 30mm thick by 40mm and about 40cm long
a circular saw(with guide)
ok first thing you need to do is off one end, mark 2 lines, and as for all things the marking out is pretty important, one at 10mm and another at 20mm(+your saw thickness, so if your saw has a 3mm cut mark the second line at 23mm)
using your guide cut the first srip as neat as you can, then once cut flip it so the manufacturers edge is now facnig the inside of the board and lines up with the second line you drew exactly then screw it on.
this should now be square with the long edge of the board. it will be where your glass rests up against to measure and cut.
ok now for your slide, similar to your first cut but from the other end just mark and cut 2 pieces, the first should be 15-18cm width and again flip the edge so it faces the inside of the board.
then cut another piece(being straight doesent really matter) at roughly 10-12cm.
now is the fun part, simply put your two second strips about 20cm appart and screw the strips of timber onto the ends.
to do this sit your strips on the board and when you attach the timber have it so it hangs over the edge on both sides(lengthways) on your board.
sorry if this is a bit confusing without pics but you should basically now have a slider that can traverse the length of your table.
then all you have to do is cut two little notches about 8mm accross and about 1-2.. below the edge of the board into the timber bits so when you score your knife line you can drag the pen over both the edges of the glass all the way..
bobs your uncle, if your making lots of tanks the same size you can simply mark lines on your board and line your cuts up with that, by securing the glass hard up against the squared edge and the closest edge of your slider every cut should come out square nice and easy
if you dont get any of what i have said let me know and maybe ill make a small model of what i mean...