I don't know much about aquariums but it sounds like you should use a Netduino with tcp/ip onboard, like the N2+ (or N+). The Netduino could then use its ADC to read temperature from a submerged temp sensor. You would then typically build a Web Service to run on the PC and have the Netduino post readings periodically.
Depending on how many readings you want to be able to store, you might need an SQL database on the PC.
If you got the PC close to the aquarium, you could use a USB to serial converter to transfer data from the Netduino.
As for the rest, I think you need to explain what you aim to do in more detal:
What other sensors are there to read - water level, etc?
What things can you control - air pump, heater, fresh water inlet, etc?
Also, there are a few other threads here on the forum where aquariums are discussed, like this one: