If testing both Nutrient Strength and pH, the nutrients are still best changed each week. However, Nutrients need only to be changed on a fortnightly to four weekly basis if desired.
The more often you change your nutrients, the more 'balanced' your solution will be. But as you will be adding nutrient as you go, the nutrient balance does not become as critical as it does without adjustments.

Testing
For pH control you will need pH liquid dropper kit, $10, pH test tape (1mtr $10, 5mtr $40) or a Waterproof pHScan1 meter $110, pH Raise $7.50 and pH Lower $7.50.

For Nutrient Strength control, a waterproof CF truncheon meter is $135.
It is recommended to get a waterproof meter as they won't corrode inside from the humidity around the tanks.

A simple lesson on meters
If price is a stumbling block, then don't buy any. Use our dump and change system outlined above. It's the best way to avoid problems.
Too many times we have heard of growers stories of the evils of buying cheap meters.
For instance, a meter might tell you the pH is 7.0 You adjust it down to pH 6.0. However, the cheap meter made an error, and it was actually 6.1 pH. Now you've adjusted it too far. The plants are sick, so you bring a sample to our store and have it tested. Its actually 5.1 ph. Aha you think and try to get a warranty on you cheap meter, which goes off and comes back again a week later, with the test saying, Calibrated OK. But you don't want to have that meter back, and you yell and scream that it made your plants sick! In the end, you have to buy a good one, or use none at all.
First lesson: buy cheap, buy twice.
Second lesson: If the plants look alright, they probably are. If they look sick, don't trust the meter automatically. Look for a reason and ask for advice. That's how to learn.

Note on reliability
We have tested expensive meters, and cheap meters.

pH meters need to be checked as the calibration can go out, but overall, any pH meter under $100 usually gives us a high return for failures. More expensive meters have longer lasting probes, however the pHScan1 WP has a probe that can be serviced (unlike most expensive meters) and has a replacement probe priced at $50 which is very cheap. I have reservations with the more expensive meters being less than completely waterproof, as humidity in Queensland and in growing areas generally can corrode the internal circuits of electronic equipment.

CF testers can be alright under $100 but can have a problem or two. The CF Truncheon has the same probe they use on the $3000 dosatronic computerized Nutrient dosing system. It's the best seller in Australia, New Zealand and USA. There is no advantage to being more accurate, or spending more money. In fact, spending more money usually involves stepping away from waterproof meters and since growing areas have high humidity, I think this is a mistake. I have two $600 meters that I just don't trust because of corrosion over time.

For pHScan1 WP meter specifics we suggest
http:\\www.eutechinst.com/products/testers/wpphscan12.htm
For CF truncheon go to the http://www.nzhydroponics.com

Calibration.
electronic testers will need to be checked and adjusted for accuracy every week. A calibration solution is essential to test your meter - however, when you buy your meter, the meter should be tested for you so you can learn how to do this. Replace your solutions regularly. pH 7.00 solution 250ml is $5.00, and CF 27.7 250ml is $5.00 also