I would take a DSLR over a point and shoot any day,
Shutter lag, slow focusing, lower noise at higher ISO, and the ability to buy different lenses that match your shooting style are only few of the factors for comparison.
As for the tricks it depends what you are shooting, for macros start by using a tripod, lower Iso values, shoot using your camera timer or the remote control shutter release, try to shoot vertical to the glass, cut off your water circulation to decrease chances of motion blurr. Personaly for macros I like to shoot in aperture priority modethis way I decide according to my composition: do I want shallow or too much Depth of field? If the aperture chosen causes a very slow shutter speed then I would bump the Iso a bit up to make the shutter speed a bit faster. try to experimemnt with different WB sometimes cloudy works best in tanks, if not try the other ones and maybe also try measuring it by placing a white sheet (or neutral gray one) nuder your lighting and measuring your white balance from there. For fish photography I would use Shutter priority mode and chose at least 1/50 or 1/60 (this will differ according to which lens you use usually shutter speed should be at least 1/focal lens used) if your handholding your camera, you can go slower than this using a tripod or supporting your camera on the glass, and try to shoot while fish is not moving. I guess those should be good starting points then assesment of your images will pinpoint areas for improvment.