Less time spent messing around with your tank is more time spent enjoying it, or spending with your loved ones, or doing whatever it is that you need to do in everyday life. Testing takes time and since my tank is pretty dialed in, I don't like to waste that time testing everything on the planet. But in order to have a really successful tank, you need to stay on top of things and really get to know your tank. Part of this does involve some testing. So the goal is to find that happy medium where you are not ignoring your duties, but the tank doesn't own you, either. There are plenty of "number chasers" out there. People who drive themselves crazy trying to obtain a specific number for things like phosphates or nitrates. It's good to have goals and be proactive, but where did these magic numbers come from? Are you blindly chasing them or are you paying attention to what your reef is telling you? Find that balance and not only will you have a happy reef, but you may be able to relax and free up some time.
For me, Alkalinity is my go-to test, testing every few days. It's the only thing I test regularly. I like to observe my tank and if everything looks good, I don't really care what some number on a test kit says (within reason). If I don't see any nuisance algae, it doesn't matter to me if a test kit says the results are a little higher than some "expert" on the internet says it should be. This is a crazy hobby, where there are soooooo many variables and different ways to skin a cat, that there never seems to be one single solution to a problem or one way of doing things. So as I said, I like to observe my tank and let it tell me if something is off. If I do start seeing nuisance algae, and now my test kit is showing levels higher than recommended, well then now I will start to think about making some changes to correct it. But the key in a reef tank is STABILITY. Coral, especially SPS, do not like swings. So there is a balance or an efficiency to be reached, where you know what to test for and how often, while not spending all the time you have for this hobby staring at little test tubes of colored water. Much of this will be learned over time as you get to know your tank and as it evolves and matures. Below are my thoughts on the most common tests and what I feel are the ones you should devote your precious time to and some others that I feel you can ease off of.
Salinity
This should go without saying but I'm putting it in here just so you can't say I didn't mention it. ;-)
Please buy a refractometer and calibration fluid. If you don't have a conductivity probe, check often. If you do have a probe, check occasionally just to make sure your probe isn't giving a false reading.
Ammonia
Are you just setting up your tank and going through your cycle? Fine. But once that is complete, you can put your ammonia test kit in a drawer and fughettaboutit. During your "cycle" the Nitrogen Cycle is completed. Without getting all technical on you (that's not what this thread is about), during the cycle, Ammonia gives birth to Nitrite, and then Nitrite gives birth to Nitrate. During this process you will see the results on your Ammonia, Nitrite and Nitrate test kits, as the results of each rise and then fall in that order. Nitrates may stay elevated for a while. But by the end, there should be no more Ammonia and no more Nitrites, both of which are lethal to your tank's inhabitants. The only time you would ever see either of these two again is if something is seriously wrong with your tank like a bunch of dead fish. In this case, its the large die-off that is responsible for the ammonia, not the other way around. So put the ammonia (and nitrite) test kit away.
Nitrites
See above.
Nitrates
There are several schools of thought on this. Some people run tanks with Nitrates over 10 ppm and some swear there should be less than 1 ppm. I fall somewhere in the middle. I run a mixed reef, heavy on LPS and SPS coral, and I have had nitrates around 10 ppm with no apparent problems, but I do feel this is at the high end of my personal comfort zone. Ideally, I would like there to be a few nitrates, somewhere between 1-5. But I'm not going to stress out either way. Right now, mine are around .5 ppm which is a little low. I feel that if the water is too clean, there isn't enough food for the coral which can produce paler colors. Adding more fish and feeding more can help. But this is why I don't stress if I see Nitrates hovering closer to 10. Basically, I'm fine with it anywhere between the two. I don't test it very often at all, maybe once a month, if that. Pay attention to how your coral respond at different Nitrate levels and over time you will figure out what's best for your tank.
Phosphates
This is another point of contention amongst reefers. Some are adamant that you should have PO4 that is .03 or less. Others run their tanks with PO4 over .1 with no problems. I fall in the latter camp but there is no real number I'm shooting for. I basically don't worry about it either way. I have had phosphate measure around .13 and I recently tested and it was .01. Honestly, I don't trust the Hanna Checker (this is what I use to test) more than as a rough guide. If my P04 is anywhere between zero to .15 I'm perfectly fine with that. As long as the tank is looking good, no nuisance algae, I test this once every couple months. Really just out of curiosity.
PH
I have a probe for this, hooked up to my controller. I never use a liquid test kit unless I think something is seriously wrong with the probe. And this is one test where daily swings are completely normal. If you have a controller, you probably have a pH probe. If you don't have a controller, get a stand-alone probe and put it in your sump. Keep an eye on it just to make sure you don't swing too high or too low. My pH tends to swing from around 7.7 in the morning to a little over 8.0 at night. Perfectly fine. Mine tends to run on the lower end because of the C02 from my Calcium Reactor. Yours may average a bit higher.
The Big Three: Alkalinity, Calcium & Magnesium
This group is the most important for a reef tank. But what you need to test depends on how you control these three elements. When you are just starting up, you won't need to dose anything because you don't have enough (or any) coral in the tank that are growing and using up these elements. For a little while, water changes should be enough. What throws a little bit of a wrench into this, is that each brand of salt has different levels of these three elements, so you should pay attention to that and use a salt mix that has an even amount of these elements, close to natural sea water (NSW). What you don't want for instance is a salt that is really high in Alk but has Calc at NSW levels because this throws off your balance. It is very important to keep Alk and Calc at balanced levels. An imbalance here can start causing issues with your coral and it can be a bit of a pain to get back in balance. And even more important than the actual number is STABILITY! For instance, Alkalinity of somewhere between 8.0-8.5 dkh is a good starting point, and keeping it there is very important. If you start bouncing around from 8.0 to 9.0 to 8.2 to 9.4 your tank is not going to be happy.
I like to test these elements once a week in the beginning. Eventually you will notice levels starting to drop. Your coral are growing and they are using up these elements faster than they can be replenished with water changes. You will need to pick a way to add these elements back in. One way is with bottled liquid additives available at your LFS. If you choose this route, I'd go with whatever is as close to the purest form of these elements as possible. But I'm personally not a fan of these mystery liquids. I recommend three different avenues; Kalkwasser in the ATO (or a Kalk reactor), Two-Part Dosing, or a Calcium Reactor. Reasons for using each is an article unto itself, but which you choose will determine what and how often to test. Both Kalk and a CaRx add Alk and Calc in balance amounts. So for the most part, I just test Alk. And once things are dialed in, I test it every few days. Maybe once a month I'll test calcium just to make sure. With two part, I tend to test the calcium a lot more often. You should be adding both in balanced amounts but since they are separate I like to make sure things are level. I now use a CaRx, so I almost never test Calcium. A CaRx is dissolving dead coral skeleton just like in nature so there is really no need to test both. Alkalinity must be kept stable to have the most successful reef tank, especially if it is SPS heavy. So i make sure that my Alk stays pretty rock solid and this is why I test it 2-3 times per week now. I would be willing to give up every single one of my other test kits but you would have to pry my Hanna Alk checker out of my cold dead hands.
What about Magnesium? It gets used up very slowly compared to the other two. BUT... it's very important because if you are not paying attention to it and it drops, it can cause the others to precipitate out of the water without being absorbed by the coral, and levels will drop even though you are dosing these elements, effectively wasting them. So check your Mag once a month or so just to make sure.
Conclusion
So that's my thoughts on testing for a reef tank. Be smart about what you really need to test for and what you don't. Spend less time playing with chemistry and staring at color charts, and more time observing your tank. And stop chasing numbers! Get to a point where things are stable. Observe your tank. Look for things you want to improve, do your research, and from there make small changes. If making a change, do it slowly and then keep it there for a while, while you observe again. And for those tests you are doing, what about keeping track of them? I mean if you are going to do the work, you might as well keep some kind of log so you can look back and see your tank's history or at least know what your previous results were. I really like using AquaticLog for this. You sign up and create a profile for your tank and you can track pretty much everything you need to. Test results, fish and coral, supplies, etc. And that's just the tip of the iceberg. Post photos, check out other people's tanks/profiles, get reminders for your tank, and you can input info from your phone, right in front of your tank... it's great resource and can be a real time saver for the Every Day Reefer!