Slave Flash too much?

You can try diffusing it or you can also try adjusting your shutter speed.

nice tank! wish i could help...just figuring all that cam stuff myself atm

Diffusing the light will defintely help. Also maybe try taking pics when it's dark. Looks like there's a lot of ambient light in the pic. I also just noticed that there is a flash reflection on the last tank pic. I take it you are also using your built in camera flash as the master and it is setting off the slave? Problem is your flash on your camera is pointing right at the tank, this kind of negates your flash placed on top of your tank. It is better if the flash source is coming directly above the tank and pointing downwards instead of the flashing pointing directly at the tank. You'll need to find another way to set off your slave flash.

The best solution to your problem would be to find or make a soft box for your remote flash unit.

Being as the closer the light source and the larger area it takes up the better and more natural the lighting will appear, the soft box solution is ideal for tank photography. Here is a simple way to make one.

Buy some ¾ inch PVC and four elbows for the corners. Measure and cut the PVC to make a frame that will cover the top of your tank. Buy some thin white cloth or use a piece of bed sheet and cut it about three inches larger than the frame. Wrap the cloth around the PVC frame and sew it. Place the frame over the top of your tank and position the flash head about two feet or so above your newly made frame. Cover the flash head with a black cloth of some type (if you're not using a sync cord to set off the flash and relying on a slave, make sure th black cloth leaves the slave sensor exposed} and allow it to drape over the PVC frame so no light can escape. Bingo you have a nice soft diffused light that will give a very natural look to your tank pictures.

Depending on the intesity of the flash, you may need to use several layers of white sheeting material. This can just be placed above the existing frame if needed.

Cool tank.

I find it easiet to not use flash at all, and just ramp up the tank lighting. I keep a VHO compact floursecent fixture just for this purpose..