To start with, i would be very careful about what to stock your tank with. 25 litres is very small and you are limited on choices. By stock i mean what fish, snails, shrimp etc you plan to keep.
Danios are certainly not a good idea. Even the very small ones like celestial pearl danios are active swimmers and will need double what you have. I would say the same for minnows. Your tank is big enough to support those types of fish, but they wont be happy in such a confined space. The problem is that the small fish you are looking at are generally social fish and need to be in groups, and your small tank wont support those numbers or give them swimming space.
If you want a recommendation for a fish in a 25 litre, consider a single betta. They are small enough to live comfortably in your tank and dont need company. You could add a snail if you wanted to see something different along with it. If you really want those types of schooling small fish, get a bigger tank. 40 litres minimum, preferably 80 litres. Bigger is normally easier.
As to your water parameters, your pH and nitrate are high. Are they that high from the tap?
I wouldnt add any fish until you can test for ammonia, and given your high values i would check both your tank and tap water.
As to test kits, liquid tests are more accurate, easier to read and as you get 100s of tests from a liquid test kit, much more cost effective in the long run. Its a good investment. I would rather invest in a liquid test kit than buying some ammonia test strips, but its up to you.
There are essentially 2 ways to cycle a tank. Fish in, you stock lightly to start with, and control toxic waste through water changes, and gradually increase stocking over an extended period of time. You can either determine your water changes through testing, but if you stock lightly and change some water frequently, your tank will likely cycle in the background without you ever noticing.
Fishless cycle is where you replicate fish waste by adding an ammonia source. Either pure ammonia, ammonium chloride, or some people use fish food or a cocktail shrimp. I dont like the fish food/shrimp route because you have no idea what level you are raising ammonia too, especially if you have never done it before. You keep dosing ammonia until your system develops enough beneficial bacteria to for your cycle to process out the waste, and you know when this is through regular testing. This typically takes 3 to 8 weeks. Once done you can add fish into an already cycled tank.
Which do you prefer?
I would get something to test for ammonia, and let us know what this is in your tank, along with results for your tap water.
