Shoot in manual mode. Remember, memory is cheap and you can always delete the bad ones. If you are not completely comfortable in manual, try aperture or shutter priority to start. Should be marked with an "A" or "S". Manually focus it for macro work.
Use a tripod, turn the pumps off.
The pixel effect is from probably too high ISO setting. 8MP is MORE than enough for good quality pics. You need to set your ISO to 400 or so, 800 will work too but will cause some graininess.
Read the manual too. It probably has some good pointers.
Check out the book "Understanding Exposure" by Bryan Peterson. You have to have some understanding of photography
(if ISO, aperture, shutter make no sense, check out the Digital Photography for Dummies) to get the most out of it but it is a nice read.
Do you have a local camera shop(not Ritz or Wolf) that has beginner classes? Might be worth checking out. Also, continuing education classes at the local college.
That 200mm lens should be able to take some decent "pseudo" macro shots. You might be 3 or 4 feet away from the tank but it will work.
Go out and shoot. Look at the results, make a change in aperture/shutter speed/ISO, analyze the change it made to the picture and repeat. Make one change at a time though. You can even do this in your house taking pics of the SO/kid/pet, etc. Play with it man!
