I’ve got small black gravel.
Cardinals are fine for a 54 litre, in facts it’s more than enough room for them!
Could I go for 12 cardinals and 6 harlequin rasbora?
I am going to disagree with the bolded portion of this post. It is
barely enough room for them.
http://www.seriouslyfish.com/species/Paracheirodon-axelrodi
An aquarium with base dimensions of
60 ∗ 30 cm or larger is recommended.
You'll note that that is the minimum recommended tank size for a group of cardinals. As such, you are going to find more problems keeping them than if they were in a larger tank... one in which there actually
was 'more than enough room for them'.
Adding more individuals may in fact help them to feel more comfortable. And, while seriouslyfish does recommend these fish to be kept with other peaceful shoalers for their best display, the balance of the aquarium must also be maintained.
1 - There is not a lot of swimming space, so a second mid-water shoaling fish will not work best.
2 - Bioload must always be considered in a small tank. Adding dwarf or micro fish in a tank this size is generally preferred, as they do indeed have more space to swim, and have a lighter bioload than larger fish.
3 - So... adding another fish species to this tank is not a great idea, if the fish is a mid-water swimmer.
4 - A dwarf species corydoras would be a nice addition, but they need a fine substrate. Please elaborate on the 'small black gravel' you mentioned. I will state, without knowing the fullness of the situation, that the substrate you currently have is likely
not going to be suitable for the corydoras... sadly.
In the meantime, I'd remind folks to stick to the topic and not delve into the motivations of different members.