Sounds like you are not using your macro or you don't have one. Most digital cameras have a macro now. There are two things you can try, but even as a photographer its hard to tell you what may be the real problem without actually being there.
First: try backing up a good five to six feet from your tank and then push (zoom) in. If you still can't focus on your object (pick something stationary first) then just back up a little more. It should come into focus.
It can be a little difficult to take a photo from that far off but it is very possible and looks great.
Second: look for a macro on your digital camera. Some digital cameras show the macro as a flower icon. On a manual it will be a ring close to the mounting assembly for the lens. When you use a macro lens it allows you to focus on things that are normally too close.
Originally Posted by mr funktastic
i am having a very hard time focusing. it seems the only way i can focus is if i am on the infinite focus or i other words no zoom.
When you pull out all the way ("no zoom") your depth of field increases dramatically. Your depth of field is the area that appears in focus through the lens. You ALMOST don't even need to focus if you have pulled out the entire way.
I hope this helps you guys. Try it and let me know what happens.