I learnt the hard way. Do not take short cuts with tank repair. I HAD a second hand 1220x650x700h 500lt tank with a chip on the bottom edge and a small crack(about 120mm long) I was given a large piece of 10mm glass to cover over crack, Siliconed down , wait 2 weeks filled , waited a couple of days ... no leaks sweet! Set up with rocks ,coral, etc ,, added fish over the next few weeks , breeding going on every ting fine.... then....drip,drip, drip, bugger!!! Long story short many lost fish ,ruined stand , and a new run from the chip out and along the middle of the bottom all the way to the end . Should have replaced the entire bottom..eh, The first crack was no bigger though! New tank looks nice.