the fish store is going to charge you big time because it says safe for aquarium when its the same dang stuff. 100 percent silicone is 100% silicone is 100%silicone.
Right now I only have a 35gal up and running. I wanted a larger tank and larger fish so bad it was killing me. Then I found the 140 that needed work and I ended up getting it because I have faith that i can fix it. It will be a lot of work but after it will all be worth it. I want to get some cichlids (not sure what kind yet) but I know I was South American cichlids.
to re-seal A tank takes allot of prep, 90g took 9 hours strait work, start to finish. google DIY on it and cut no corners, if you feel suspect on something take the time to make sure its not.
The reason those silicon say not for aquarium use is cause if you use it in a fish tank but it fails then they don't have liability. Tbh as long as you take time and do the reseal correctly most silicon are OK. Some are better than others, GE 1 gets used a lot, also Dow is good.