The glass walls of the tanks are going to play havoc with the rooms accoustics.
As you know, the general reason for the suggestion of curtains over glass is to absorb the sound that would be reflected from the glass.
But, you have one good thing going for you in this situation, sound is absorbed by mass, and your 200+ gallons of water weighs well over 1700 pounds.
Thats some mass!!!!
And while the sound is going to reflect from the glass surfaces, the amount and frequency of the sound will be far different than the typical glass window. Why? Because high frequency sound waves are readily absorbed by water. A good example of this would be to tap an empty tank with a metal spoon, and then the same with a tank filled with water. The empty tank rings with a "high" pitch, while the full tank has an attenuated low pitched "thud".
My only suggestion is diffusion in the area of the tanks. A lush tropical forest of plants around the tanks could possibly work (if the leaf structure was dense enough), and the fish would not mind the change of scenery.
The concrete walls will also be a potential source of reflection problems.
It may be easier to dampen/diffuse the reflections from the concrete walls ( I like bookcases with lots of books and stuff), in which case the reflections from the fish tanks may not be as detrimental to the sound as first thought.
Remember, we don't want a dead sounding room, rather a room with a balance of absorbtion and reflection.
Speakers, some speakers, such as the Klipsch, have a bright, vivid sound, that may not perform well in a bright lively room. That said, the setup equalizer on many receivers can solve that problem, so the best advice is buy what you like.
I would look to purchase a better sounding/performing 5.1 system, rather than a 7.1 system. The mid sized room is better suited to a 5.1 system, and the best advice I have read is that a good 5.1 system will always perform/sound better than a lesser quality 7.1 system, especially in a small or midsized room. Besides, you could always add the 2 extra speakers at a later date.
By the way, I own Polk LSi's. My previous sytem had 20 year old Infinity's. If you purchased a Polk LSi system, it could set you back $2000 or more. The LSi's could reachable within your budget. But whatever you do, all the speakers listed are of good quality, have a strong reputation, and many loyal (repeat) customers.
