is it a plexi tank that you are putting dividers into or a glass tank you are putting dividers into, remember that silicon will not stick to smooth plexi forever, you will need a plexi glue bond if you want a permanent bond between 2 plexi peices, you can get that at a glass shop that sells plexi as well. I know a guy who DIY tank and made it with plexi and silicon (cured for over 2 weeks)and just as the tank was filling up it burst apart, lucky he was outside when he tested it. Figure if it needs to be water tight or hold a load you will need to glue bond it, if it is just a divider and no big deal you can silicon it but I would not recommend it unles for testing prior to the permanent bond.