One of the tricks to getting good pictures through glass is controlling where the light is coming from. You want more light behind the glass than in front of it. You might try hanging a tarp up to cut down the outdoor light getting to the front of the glass. Or the more expensive route to take is to install some really bright viewing lights above the water.
Now keep in mind, my photography gear is a lot more than a regular point-n-shoot camera. I use a number of lights coming in from different points to achieve different objectives. But my investment is primarily in lighting, not in camera & lens.
I have a light to the right, a light to the left, and a light behind the model.
The light to the right is really bright. The light to the left is a soft fill to diminish shadows. And the light behind is crisp and hard to separate her from the background so she doesn't disappear into blackness.
If you have portable halogen work lights, you might be able to achieve something like this effect with your fish. Put one light behind the tank. Place a couple more around the tank. Put a tarp behind you and the camera to block out natural sunlight. You might have to experiment and move the lights around until you find the right angles.
Just playing with the lights can produce
dramatic differences. Here is the same model with a different lighting scheme.
Note the strong shadows and lack of background separation. I was going for something like this, but it's not so good for fish pictures.
That tank is awesome. I'd love to have something like that in my back yard!
I wonder if it would help the algae to have an open loop system running through a hydroponic garden outside. Let the plants suck up the nutrients that would otherwise go to the algae. The term to google for more information would be
aquaponics.