On the tank stocking, I think the 25L aquarium is too small for platies and tetras (in which you kept before they died). For most tetras, you would need a minimum 15 gallon long (60L) aquarium and over; and for platies, a 20 gallon long (80L) is preferable since they are livebearers.
@Fishkeeper123 is correct on this. Either small snails or shrimp for the 25L, but I've heard that sized tank is the absolute minimum for a single male or female betta. However, for the fish to be happier, he or she would need to be kept in a 10 gallon (40L) long aquarium because it's more spacious for the fish to swim. Other than that, the 25L would also be used as a temporary hospital tank for smaller fish.
AqAdvisor is a good stocking calculator if you plan on getting fish again: