IMHO, Dallas
1-wire devices would the the cheapest and easiest. Sounds like you have a PC already and your favorite database.
First, you'd need a
USB 1-wire adapter.
A
1-wire temp sensor encased in something waterproof, like epoxy. Sensors are cheap, so add as many as you want. I even have sensors in my MH ballasts, in case they overheat.
You would of course need a pH probe. A pH transmitter would be needed to read the probe and convert it to a voltage that can be read by a
1-wire A/D converter. The pH transmitter would need to output a 4-20ma loop. Google will yield lots of information on 4-20ma loops. It's a standard for industrial controls. You can find pH transmitters on eBay pretty cheap. I got one for $25. Even brand new one can be had for around $80.
Some CAT-5 cable can be used to connect the sensors to the USB adapter.
You will then need to choose which
1-wire API to use to get the data. If you are just reading the sensors, then the java API and some batch files would work. You can use a java program if you want to get slick. The pH would need a bit more calculation. A point-slope formula would need to be used to derive the pH, based on the voltage of the probe on a pH 7 and voltage at pH 10 calibration fluid, and of course the current voltage.
There may be cheaper ways, but they would probably take much longer to do.
As far as X10, I have it but would never use it for my reef. I don't think it's reliable enough, but some people do swear by it.