My suggestion is to explore the impact of the exposure metering, aperture, shutter speed, and ISO settings if you haven't already, before you look at the SLRs. DSLRs are great for a number of reasons, but you might be able to take tank pics without any problems with the right settings on your existing cam. It looks to me that the overall exposure levels are different in the two pics above, and it might just be that you need to manually adjust the camera settings when you shoot your tank. I'm not familiar with the CP4300, but I just found some specs on it and it looks like you should be able adjust the settings manually.
ISO rating Auto, 100, 200, 400
Zoom wide (W) 38 mm
Zoom tele (T) 114 mm (3 x)
Aperture range F2.8/F7.6, F4.9/F13.4
Min shutter 8 sec + Bulb
Max shutter 1/1000 sec
Exposure compensation -2 EV to +2 EV in 1/3 EV steps
Metering 256 segment Matrix, Center-Weighted, Spot, Spot AF Area
Aperture priority Yes
Shutter priority Yes
For the blurry pics with the bokeh effect that you asked about, you want to set the aperture to a wide value / small number, like f/2.8. That will reduce the depth of focus, and you then need to adjust the ISO and shutter speed parameters so that you get the exposure level right with a shutter speed that's fast enough that the fish don't appear to move in the image.
To take sharp pics and reduce hand shake, you want to use a tripod and the timer (or a shutter release) on the camera so that you're not accidentally moving it by pressing the button to take the pic. Whichever route you go down, a tripod is an important accessory.
In your pics, it's hard for me to judge whether the over exposure on the sand is due to limited dynamic range on the sensor itself or that the pic is just slightly over-exposed because of variability in the exposure metering. It looks like the overall exposure level is slightly different between the two pics, and it might be due to the exposure metering mode you have the cam on. As you're playing around with the iso, aperture, and shutter speed, you should also look at the overall exposure compensation (as well as teh exposure metering mode) and see if dropping the exposure down (ie, intentionally making the pic slightly dark) will help reduce the sand being over exposed.
HTH