Yes, I concur with 1077, and will expand on a couple things that stand out to me from your info.
When new fish are introduced to any tank, they are under severe stress--from their initial transport to the store, being kept in what are almost certainly poor conditions in any store, then being chased and bagged, then undergoing a (usual) major change in water quality from bag to tank. Stress weakens the immune system in fish just as it does in humans, making them even more susceptible to any even "minor" issue in the tank. The stress caused to many fish by the presence of fish like Tiger Barb [which send out aggressive signals chemically even if not physically] can have an impact. All of this can play into things.
You have the space in a 125g, so if you like the Tiger Barb, get at least 12 (in total) of them; the info in our profile explains this. [You can click on the shaded name to see that fish's profile.]
The Serpae Tetra is very similar to Tigers in their behaviour. Please read the profile. Increasing the group to 8+ may (or may not) resolve these issues. And btw, while the fish may appear OK now, as they settle and mature their natural instincts may become more pronounced--not always, but often, so it is best to recognize what is likely and be prepared.
Forget rummy nose tetra. They need a large group too, 12+, as noted in our profile of the Brilliant Rummy Nose Tetra [the most commonly-available species now]. But these will be wild caught fish, and in anything but soft (very soft) acidic water, they will not last. They will also be stressed by the aggressiveness of Tigers and Serpae; I personally would never mix these, the rummys are too beautiful and wonderful a fish for this.
Other tetra and danio--all are shoaling fish, requiring a group of at least six but with space more is better. Shoaling fish need their companions to reduce stress, and many have interactive social traits within the group.
I would consider removing the Chinese Algae Eaters. When young they eat algae, but as they mature they eat less and less and almost always attack other fish. They attain 6 inches, so they are a force to be reckoned with and can cause havoc to the shoaling tetra, etc.