I'd find out if it's tempered first and if it is I'd walk away. I've cut a few tanks apart in my day and you always end up chipping the edges, even if only slightly. The problem with tempered glass is it won't shatter from small chips but there's a possibility the internally stressed glass structure would be compromised meaning it could blow out later, and with 900 gallons behind it, it's simply just not worth the risk.
With that said cutting the tank apart isn't that hard. Remove inside beads first as well as any bracing if present. Plastic bracing will likely have to be scarified. Cut side seams first then pull the glass back to break the seal on the base, finish with knife. I'd use an x acto knife, you can actually get sets like
this which work well. Getting the blade in can be tricky depending how the tank was assembled but if you start at one side and use some leverage you can usually work your way through the seam with ease. Personally never had much luck with piano wire.