Sounds like a job for a SHT15 - http://www.sparkfun.com/datasheets/Sensors/SHT1x_datasheet.pdf - in combination with a softserial library .
One Arduino can sample 4 sensors - once per 10 seconds or so - ; add an ethernet to post all the sensor data to a PHP page on a PC which stores it in a MySQL database or a logfile per sensor per day or so.. You can use this data to make nice graphs
Furthermore add 4 outputlines for solid state relays to switch PC-fan's for fresh air - they need a separate 12 Volt. (the automatic cooling is kept locally this way, independant of the logging PC..
A warning system can be built in the PC if the sensors are not logged, if temp or humid is too high or too low.
Please consider at least a master temp sensor that monitors the room temperature too.
my 2 cents,
Rob
SparkFun Humidity and Temperature Sensor Breakout - SHT15 - SEN-08257 - SparkFun Electronics - SHT15 - $38 if 10+
http://www.sparkfun.com/products/9950 - Arduino UNO - $27 if 10+
Arduino Ethernet Shield - DEV-09026 - SparkFun Electronics - ethernetshield - $42 if 10+
DC Brushless Fan - 80x80x15mm (5V) - COM-09649 - SparkFun Electronics - fan - $6 if 10+
So ex cables and power supplies ~70 + 4 * 50 = $270 for monitoring four tanks Adding cables (especially ethernet) etc will add extra bucks so in the end it must be makeable for $100 per tank for the hardware