1) Your tank is quite well lit
2) You're using about ISO 100/200, so noise on S5is should be minimal
3) Shutter speed is about 1/100 - 1/200s
I think these are decent (almost ideal) conditions. I don't see why the S5is should be hindering your fish photography in any way.
Furthermore with the prosumers like S5is, macro function is built-in. You don't need to purchase an additional macro lens to get up close to the fish, as you would require with a DSLR.
Try experimenting with better lighting to get more interesting effects as well. Don't immediately jump on the DSLR bandwagon.