Yes, 25 fish would be too many unless they were all less than 1cm long at adult size. Some shop workers have no idea what they are talking about. A lot are told to make things up to get a sale
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With a tank with that footprint, you will be limited as to the fish you can keep. Most bottom dweller, for example, are not a good idea as they need a big footprint to live on. As I've mentioned before, red or yellow honey gouramis would be suitable. I find the yellow ones easier to tell the sex. I can tell you how when your tank is cycled - I have a trio of yellow honeys in my 125 litre
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The tank would have had no ammonia or nitrite in it this morning as there were no fish in it. As soon as you put the black neons in they would have been making ammonia, so that will be going up. The plants should help but only to a significant degree if there are a
lot of them and they are growing quickly.
You may already know all this, but I always assume a newcomer to the hobby, or one like yourself who hadn't had fish for a long time, is unaware of what happens when fish are put in a tank. If you do already know, ignore all the following
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The first thing you will see is the ammonia level creeping up. You need to keep this down below 0.25, and as close to zero as possible. As you tested after the water had been in the tank a few days, your pH is a 'stood' one (I'll explain that in a minute) so it will remain at 7.2. Test kits read both ammonia and ammonium. These both exist in the the tank in an equilibrium, and the exact amount of each form is pH dependant. The higher the pH, the more ammonia. Ammonia is much more toxic than ammonium, and at your pH you'll have more of the toxic ammonia than someone with a pH of, say, 6.8. So it is more important in your case that you keep the ammonia low by doing water changes.
For several days, perhaps a few weeks, you will find the ammonia reading has gone up every time you test, and you need to do as many water changes, and as big as necessary to stop the ammonia reading getting above 0.25.
Eventually you'll notice that the ammonia reading is going up more slowly and eventually staying at zero. This is when you'll find nitrite showing up in the tests. You need to keep this below 0.25 by doing water changes, like with ammonia. The nitrite eating bacteria take longer to multiply than the ammonia eaters; as a rule of thumb, twice as long.
For a couple of months, you need to test at least once a day for both ammonia and nitrite, and do a water change as big and as often as necessary to keep both of them below 0.25 at all times. It is worth making a note somewhere of the readings and how often you do a water change so that you'll know when you've gone a week with zero for both and not had to do a water change. That will be when the cycle has finished.
pH - it is always a good idea to test water that has stood for 24 hours (which is what you tank water had done when you tested it). It is common for water companies to pump carbon dioxide into the mains water to reduce the amount of limescale building up in the pipes. Carbon dioxide makes the water acidic. When tapwater stands, the carbon dioxide gasses off and the pH rises. As your tank water had stood before you tested its pH, that is a true reading for your tapwater.
Can I ask, what type of nitrate tester do you have? Nitrate is notoriously difficult to test without very expensive lab equipement; it is easier to regard the reading on home testers as just ball park. But to get anything like an accurate result with liquid reagent testers, one of the bottles has to be shaken extremely well. All makes of liquid nitrate testers have 2 or 3 bottles and a reagent in one of the bottles settles on the bottom. The shaking in the instructions is necessary to get that reagent back into the liquid. For a bottle that has not been used for some time, it is also a good idea to tap it on the work surface first to break up any lumps of sediment. Again, applogies if you already know this, but you'd be surprised how many people do not follow the instrcutions regarding the shaking.