What size tank is this ? Is this a front panel or side panel? Silicone won't work.
I would use weldon 3 or 4 and a needle tip and fill all those cracks.
Then get a piece of what ever size material the tank is made out of, and put that over the entire area. I would use Weldon-16 along all the edges (in all directions) put a weight on it to hold it down. And then possibly shoot some 4 in underneath to seal the sandwich.
Then 24 hours later, put the tank outside, fill it with water and let it sit for at least 3-4 days. let the sun hit the tank and cause the acrylic to heat and cool from Day to night. This will help make sure that the different materials are properly mated and won't pop.
Dave B