So I have/had a 55g four foot that sprung a leak on me a few months back, I left it for a while completely dry so the silicon would shrink, then with the help of a knife pulled it apart. I now have five pieces of glass measuring....

Front and Back piece- 122 cm x 45.5 cm
Base- 121 cm x 34 cm
Sides- 45.5 x 34 cm

The glass is 6mm glass.

I want to make a smaller tank for a Betta. I was thinking of using one of the sides as the base, cutting two pieces from the front piece 20 cm high for the long sides, and another pair of sides from the base. With the rest of the base becoming lids, the last side I could use as practice...

Is there anything I'm missing with this? I've never cut glass before but from what I've read you just need a glass cutter and a straight big metal ruler, cut a line in the glass and then snap it...

The tank was a DIY tank though not made by me, I bought it second hand and the clumsy silicone job and rounded edges point to a home made tank..

If the old tank glass is not toughened/tempered then you should have no problem cutting 6mm plate. If it is toughened it will break/explode like the old car windscreens used to do. You will only tell when you cut it as normally toughened glass would have a stamp (lazered insignia) in one corner but that would not be practable for aquarium purposes.
Always build on the base
The cutting list would be
Base = chosen length x chosen width plus 12mm
Front n Back = chosen length x chosen height
Ends = chosen height x chosen width

Depending on your cutting wheel (glasscutter) make sure you get your wheel right on the line, dip the glasscutter wheel in a fine oil first, line up you tee square or ruler (hold firm) and make one continous score along your line. dont go back over and repeat the process of scoring. Place the ruler inder the glass along the line and press down evenly. The glass should then break along the line. Rub the cut with a little fine glasspaper. Easy peasy.

When building, I construct the tank first (building on the base) just spot weld using a small bead of silicone here and there on the various panes as you work your way around. I like to position the back pane first on the base then the two ends and lastly the front, when built secure with masking tape. (you may need assistance when positioning the plates). Leave this to cure for a day. Then you can cut you silicone tube nozzel to an angle to suit the width of bead you require. Others may do this differently but I like to run two strips of masking tape down each joint where the ends meet the front and back leaving a gap of around 4mm on each pane, then I run a good bead between the two. I dip a finger in some water and press the bead into the glass. Once this is done I carefully remove the tapes. The base may be sealed just running a bead from your gun and smothing again with a dampened finger, paying particular attention to the corners. You may also consider cutting 2 strips of 6mm glass as runners to hold the coverglasses/lids. If so cut 2 pieces 60mm shy of your overall chosen length x 40mm silicone these in at the end of the tank construction about 15mm down from the top of the front and back panes. (cutting this small width will require practice (mind your fingers)
Finally once all the tank is internally sealed I like to run a very fine bead of silicone into every seam/joint on the tank exterior.
See pic below for recent small marine tank build with drilled bulkhead fitting for a sump tank.


I hope this was clear enough to help
Regards
BigC

That was really good thanks!

Still need to get around my dad somehow LOL, but theres no point wasting all the glass.

Would it be less likely for the glass to eb toughened if it was a DIY job?

Yeah, then it would be highly unlikely it would be toughened.
I was in Australia QLD in March and I was amazed at how environmentally aware Aussie folk were, Not much was discarded if it could be used again. (i.e. your glass) Even old Queenslander wooden houses were for sale in a yard near to where I was staying.
Regards
BigC

It's been ingrained into us younger generation that recycling is the way to go, besides it costs money to send that much glass to the tip.

make sure you wear glasses when cutting the glass to prevent splinters getting in your eyes.

Well picked up a glass cutter and aquarium safe silicon today, I need to wait a week til I actually do this though, as I still need to clean the glass, and the school holidays start in a week. Not sure if I'll be the one cutting it, dads liable to have a fit when he realises what I'm planning and cut it himself, which is fine by me LOL.

Not sure how I'll be smoothing the edges either... I'm thinking I might just place a fine layer of silicon over the exposed edges once the tank is made, so theres no chance of injury.

Rub all the edges with a fine grade glass/sandpaper. (Use Gloves)
Regards
BigC

PS. You know you really wanted your Dad to do the job in the first place. LOL
Tell him to follow all of the above info.....Lets know how you get on.

I use a sanding block (piece of 70x35mm pine about 4inches long) and wrap it in fine sandpaper like BigC suggests. The wood stops the glass cutting through the paper to your hands and gives you something to hang onto. Then you just rub it back a few times. Do all the edges including the corners. Then wipe the dust off with a damp rag.