First of all thanks very much to everyone who replied!
robtillaart
Excellent information!
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
So... one Arduino UNO can monitor 4 of the SHT15's and transmit the data wirelessly (thanks PaulS) via an XBee (of some sort) to a computer? Will the computer end of the transmission require another Arduino UNO, power supply (not through USB?), and XBee device(s)?
Is there a way for a solid state relay to operate something with higher voltage (regular household current) such as a: http://www.amazon.com/gp/product/B000WEIJ7K/ref=pd_lpo_k2_dp_sr_1?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B000BKGOM2&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0C34ZQGDHZT2991M5TTC
PaulS
My questions were more for getting you to think about the requirements, than to criticize anything. Writing a good requirements document is critical to successfully developing a project.
They were good questions. I didn't mean to sound defensive. Sometimes it's hard for me to communicate clearly what I have in mind to others.
As Rob's price list and wiring comments shows.....really doesn't need to be an Arduino at each tank.
I guess I just do not really have that firm of a grasp of how all this stuff works. All I need are an SHT15, XBee, power supply, and sensor wiring/casing at the tank, and with appropriate receiving equiptment (?? another XBee?) at the computer?
My assumptions that I can't possibly do this on my own are being very strongly confirmed! :-X
My intention is not to sell a system like this just to make money. If I could somehow manage to put the components together I would sell at near cost to other hobbyists. I think people would be interested in the basic function of temp/RH logging.
liudr
Nominal temperature ranges are usually between 70 F - 78 F during the day along with a drop in temperature to around 62-65 F at night. As for resolution, to the nearest tenth. I'm sure people would get their jollies from as accurate a reading as possible (hundredth).
The experiences I've had with thermochromic liquid crystal sheets is that it can be hard to differentiate between the reading as the temperature of the object it is mounted on (i.e. glass) or the ambient temperature (which is what I need)
Anyone want to make one for me? 
I would really like to learn about how to put something like this together, but I would be basically starting at square one zero. I have no experience soldering, reading wiring diagrams, deciphering very complex data sheets, programming in whatever language the Arduino uses, etc.
I don't mean to sound childish, I guess I can just be impatient.