Fish randomly dying

[charliebear78]

We are very new to this and decided it would be a nice easy peaceful hobby!
We have a 105L tropical Tank, We have Platys, Barbs and Tetras.
The Tank as been up and running now for 4 months.
We have the API water testing kit and we do a 20% water change/gravel hoover a week,Clean the filter every month using the tank water, scrub the decor etc...
We feed them flakes,dried bloodworms and on occasion peas!
I feel we are doing everything we should yet they are still dying!
We seem to lose a fish a month.
Is this normal?
It is stressing me out-I dread coming down in the morning to what I may find!
I have my faves-A green Barb named Lucky because we found him in the filter twice( we realised the filter was too low in the water) and a tiny Tiger Barb
Today the tiny Barb died-No reason I can think of-No signs of illness, swimming happily and eating well.
We have tested the water( which we do every time) and it is all good
If any fishy experts could shed some light???
I feel a bit like giving up with the whole fish keeping hobby

Just to add we have slowly built up to 17 fish which is how many we have in there now( 16 now the Barb has gone)
We Had been adding x3 a week or so,

The main thing that stands out for me is that Platys are hard water fish.
All varieties of tetra are soft water fish and I’m not sure about barbs.
Put soft water fish in hard water and they’ll get sick and vice versa.

If your tetras are neons it could be due to bad breeding.
Neons are very intensively bred and often not very hardy now.
Same with guppies.

I believe that barbs can be quite nippy so that could possibly be an issue too if they are chasing and nipping the other fish.