The main thing that stands out is the fish selection. You have some issues [kane touched on them] that I will explain.
Many of the forest fish we maintain in aquaria are shoaling fish. These species live in very large groups, and for several reasons. First, security--there is "safety in numbers." And this translates into stress when the fish are alone or in too small a group. A fish programmed by nature to live in hundreds for safety is not going to fee safe on its own or with one or two or three, and that means constant stress. Second, many shoaling fish have a distinct pecking order or social structure within a group. Your experience with Cherry Barb
illustrates this; the female is getting picked on because the male has no one else. Another female is a help but not a solution; more males and females is the solution.
All characins (tetra, hatchetfish, pencilfish), cyprinids (danio, rasbora, barbs, loaches) and many other fish are shoaling fish. Studies have confirmed that keeping less than 5-6 of most is detrimental long-term. Increased aggression frequently results, along with more stress that only makes this even worse. And stress is a prime cause of health issues because it weakens the immune system in fish, just as it does in humans.
So, before any new fish species are added, increase some of those you already have. But here we come to some issues with tank size. A 120 litre equates to a 31 gallon aquarium [some of us think better in imperial rather than metric
] which I will assume is probably 30 inches in length, maybe 36 (that would be better). Danio are all active swimmers, so they need space. Rasbora by contrast do not, they can be kept in somewhat smaller environments simply because they are less active. Water quality is obviously still an issue either way, these are generalities.
Most of the fish you mention are included in our fish profiles, second tab from the left in the blue bar across the top of the page. In posts when the scientific or common name is identical to the profile, the name will be shaded and you can click on that to see that fish's profile, example Cherry Barb
. If you have a read of the fish profile for each, you will note that it recommends 6 minimum for the tetra and danio species. Gold Barb is not yet included in our profile, but this common namer can refer to a couple of different barbs.
Obviously this is going to overcrowd your aquarium fast, so perhaps some decisions should be made on removing some species. Another aquarium (?), trade to the store, other hobbyists... I leave that to you. From your description of the aquascape, my initial thinking would be to have the loaches, and cherry barbs but add 4 more to make 7. Remove the danio species. Flame Tetra
, adding 3 more, will work with the barbs. The reason I suggest this combination is that these are all less active fish, and removing the more active danio will solve two problems and make the remaining fish more settled [= less stressed]. The gold barb is difficult to suggest until I know which species; the common Gold Barb is large and too active for this tank, the gold dwarf barb would be ideal in a group of 7.
This is just some initial feedback, I'd be happy to expand as asked. And welcome to Tropical Fish Keeping forum, since i don't believe I have had the pleasure of welcoming you previously.