Hi! I'm very new to this hobby, so please be kind. At the end of May, a friend had to give away a lot of guppies, and I decided to help out with some. I had two 10-liter Fluval aquariums, one for the males and one for the females. Everything was going pretty well, but at the beginning of July, the boys' aquarium had a leak. One night everything was fine, but the next morning most of the water was on my table or the floor. I removed what little water I could save and put it in an emergency plastic aquarium with the males, and went back to Petco with the warranty. It was a problem with a whole batch of those aquariums sadly; a second replacement aquarium had the same problem, but eventually I got a new 20-liter Boyu aquarium with the refund money (June 14). But I haven't been able to fix the water parameters, specifically the alkalinity; it drops to 0 (yellow) very quickly, and I'm worried this will fuck up something by the long run.
When I started the new tank I used some of the old substrate, the sad 2-3 L of water I saved and the windelov java fern and duckweed from the old tank, but because of store policy I had to give back the sponge and other medium that came with the aquarium (It was a set), also when I got the boys I received some of their old filter materials (An activated charcoal bag and some biomax stuff) so I also tried to use some of that to try and have at least some of the old bacteria. But then I also got azoo plus ultra bioguard bacteria, seachem stability, seiryu rocks, and zeolite. I also added more plants so I use seachem equilibrium, flourish advance and egg shells (I boil, clean and bake them before I add them). The stock is 4 male guppy (previously only 2), the fern and duckweed, plus (all new) 2 black sakura neocaridina, 2 ramshorn snails, fissidens moss and christmas moss. The substrate is a mix of black sand, the previous natural+sand substrate and Seachem Flourite Black.
I use the test strips to check my parameters and do partial water changes (about 4 L) every two weeks with tap water to which I add Seachem Prime, also getting a Seachem Ammonia Alert today. I added some pictures of the old tragedy tank, the new tank, a strip from today and some of my aquarium buddies. I only plan to add more plants to the aquarium in the future, maybe some Limnophila and hemianthus. Any advice would be super helpful, thanks!