Liquid all day. They're more accurate and you get more tests for your money. Liquid tests are a little more work. But why wouldn't you put in a few more minutes for a better result?
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That's my predominant issue right now—after being advised that my water is likely too hard and alkaline for most species by itself, I'm going to try mixing it with other (haven't decided if I'm going for distilled or RO) water to reach the desired pH, GH, and KH. However, it's starting to become a pain to go back and forth to the fish store and get my water tested several times a week, just to see if my hardness went down.I used them all, I prefer the liquid tests in general but if I want a quick result I will use a strip. With the strips I make sure I keep them very dry and don't pull a new one from the container with wet hands and they seem ok. Nitrate seems to be the most difficult for most test kits, if it seems lower than expected the test was not likely done right.
I find I don't use either much anymore because I have established my water change cycles such that I don't see a lot of changes anymore. I don't add fish or other creatures much anymore either, focusing only on my current fish, so nothing much changes. Also my fish are pretty "vocal" when things are not right, ie they behave and look different if they are stressed. For me it takes only 30 minutes to change 50 to 75% the water on the 75 gallon tank, it takes 15 minutes to go through the water tests, might as well just change the water. With a new tank I will test pre water change and post water change to assess the process, the other time I use the testing lots is when I am trying to raise fry, things can go south quickly in both cases.
On my shrimp tank I test more because I have to mix the water, but those tests are mostly GH, KH, and TDS. I use liquid KH and GH tests then use TDS probe, this gives me a ratio of TDS to the other values and I use that as a quick test. (TDS only tests conductivity so to do it accurately you have to make sure you haven't changed how you treat the tank, changes to fertilizer in particular seem to affect the TDS without changing the GH or KH).
Get a TDS monitor once you "calibrate" it to your water ie 200 ppm is equivalent to 10 GH or something similar then for the less critical phases you can just use the TDS value, measurements take seconds. They are less than an API test kit, but I would get one of those too.However, it's starting to become a pain to go back and forth to the fish store and get my water tested several times a week, just to see if my hardness went down.