Tanks don’t crack for no reason. The pressure on the glass is determined only by the depth of the water – the width has no effect. Just as well, as we’d never go swimming in the ocean otherwise. 6 mm is adequate for 44 cm high (as KJ indicated).
Is the hole at the top with the cracks spreading out there down wards? Or is the hole at the bottom with the cracks spreading out as they go upwards? You normally don’t get holes in glass without something solid impacting the glass. Nor do you tend to get radiating cracks unless there was some form of sharp, solid impact.
A plane seating surface and adequate foam to compensate for any imperfections, is very important with larger tanks. For example, if the seating surface is high in the centre, the weight of water either side is putting stress on the centre region of glass. The longer the tank, the greater will be the stress forces, even if the amount of water is the same, because it has a leverage effect. Usually there is more weight of water so that combined with the potential for leverage makes it all the more critical to ensure the seating surface is perfectly flat. A non-flat surface that you can get away with using a three foot aquarium on may well result in disaster with a three foot extra high aquarium. I learned that the hard way.
Cracking due to base not being flat, would normally occur within a week or less of you having set up the aquarium. However, if you have recently added large stones or significantly more gravel or given the glass a solid bump with a large river stone or the like, that could do it.
Another possibility is movement in the supporting structure. Fresh water weights 1 kg per litre. So your 6 ft by 60 cm filled to with 5 cm of the 45 cm height contains about 430 kg of water. Any rocks and pebbles or gravel will increase that. You could comfortably say the aquarium plus contents weighs around 460 kg. That’s the weight of 4 big ruby players. You definitely need a well built base to support that sort of weight without the possibility of long term deformation. This is why pays to have a professionally made stand for anything over a normal 3 ft tank.
I have deliberately avoided using the term level. It can be flat but not level if it is slightly tilted. In which case, the water will be higher in one end of the aquarium than the other. Obviously having it level as well is desirable but flat is the critical requirement. If you have a surface that is not quite flat, use of soft polystyrene foam or reasonable thickness followed by normal thickness sheet of firm polystyrene can often compensation for minor irregularities in the surface. Unfortunately it requires a bit bitter experience to know just what you can get away with and what you cannot. So if in doubt, go for a professionally built base.
I have been talking about wooden bases. Angle iron or rectangular tubular steel bases will overcome all the above problems and can be utilised on a wooden and with a wooden platform and polystyrene placed on them, if so desired.
Sorry about the length. I have had a couple of major disasters over the years so I know where you are coming from.
Blue