Hi upsy_daisy -- An excellent topic!
You might be surprised that technically its conceivably possible that an experienced aquarist could start and run a freshwater aquarium on nothing but tap water! If said aquarist had mature media from older tanks, it might be possible to even get by without conditioner and then to make small enough water changes that conditioner and temp matching were not needed and if the aquarist were very experienced in observing the fish and in carrying out all procedures then tests of any sort might not be needed!
What I think that tells you is that one of the main purposes of test kits is -learning- and that the use of regular chemicals, which sometimes are stongly recommended (like conditioner for immature filters,) is also part need and part learning. There is also a legitimate role for test kit use by experienced aquarists of course, to verify their premonitions and suspicions about various tank parameters!
This said, I'm sure your reading here (maybe even any number of my own posts!) has let you run across the fairly strong recommendations we have for a good master test kit and for a good conditioner! Personally, I use an API Freshwater Master Test Kit and Seachem Prime for these two functions.
The next most common set of tests are those for hardness (hardness is a term for the measuring of mineral content in water - all sorts of minerals can be in water.) I use both a Tetratec KH kit and a combo API GH/KH test kit for hardness measurements. Hardness testing is by no means universally necessary. Many can get by without going the expense of these tests. The vast majority of the tropical fish we obtain from typical sources (commercial retail and web and most local society swap-meets) are either very used to our typical tap water in our local area or are very adaptable to it. In fact, the need for special hardness considerations are often limited to fish breeding, the keeping of rare, sometimes wild-caught types and sometimes the desire to keep types of fish that really do need conditions quite far from our own tap water measurements. A good example would be mollies, which often need a lot of mineral content or they will be prone to diseases. Depending on the type tap water your location has given you and the type of fish you want to keep, you may or may not want to bother with hardness tests.
Plant nutrition supplements are more commonly used than hardness tests. You haven't indicated which continent you live in ("location" used to be a sort of automatic thing that showed up with our identity avatar etc. on the left and I miss it!) but there are different sets of current recommendations depending on that. It should be understood that plants are a big topic and that the recommendations we make here for beginners are often over-simplifying a large topic.
Given that we plan to over-simplify, if you are in the UK we often recommend a low-light-technique planted approach using EasyCarbo (or Seachem Flourish Excel, which is also available I guess) liquid carbon and Tropica Plant Nutrition Plus (abbreviated "TPN+") along with weekly water changes to "reset" the nutrient levels. In the USA its basically the same except EasyCarbo is not available so everyone dosing liquid carbon uses Seachem Flourish Excel (often buying it in larger bottle sizes to save money) and then possibly supplementing any number of things for the other macro and micro nutritents. Be aware here that its "a topic" whether one should just rely on the leftover fish waste, excess fishfood and plant debris to provide a good percentage of the nutrition(fertilizers) that the plants need! In my case I do in fact do a procedure called "reduced EI" where I actually have a whole shelf of Seachem Flourish fertilizers which I dose according to a schedule. I use individual liquid macro nutrients (Nitrogen, Phosphorus, Potassium (the famous N-P-K of fertilization)) each in its own bottle. Then I use a couple of "Trace" mixtures to provide more or less the other 13 "micronutrients." (In my case I alternate between Flourish products that are actually called "Trace" and another that has Iron but is just a straight fert mix also containing macros.) (If I were in UK I would just use TPN+!) My motto as a beginner trying to learn plants has always been: light is a skillset, CO2/Carbon is a skillset, plant nutrition is a skillset and algae is a skillset that requires the other skillsets
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Finally, you might also consider a post to Wilder in the Emergencies section re what sort of "First Aid" kit of medications to keep, if any. Personally I don't keep any but I'm not that far from some pet and fish shops.
I've probably left some things out in a rush but hope that'll get you started?
~~waterdrop~~
