Disappearing Tetras, And Now A Dead Danio

[JenLo]

Hi all,
 
About 3 months ago I bought a second hand 100L (22gallon) tank from a lady that lives nearby. It has been up and running for more than a year, and I inherited her fish:
 
2 mollies
2 zebra danio
1 pleco (I think dwarf)
1 neon tetra
 
I added to it 2 danios that I already had in another tank, then gradually added 5 more tetras.
 
Firstly, one of the mollies died not long after the move (I assumed from stress).
 
Then, two neon tetras vanished the day after I added them, again I thought from stress.
 
After a few weeks I added 3 more tetras and again, 1 vanished overnight.
 
Then 2 weeks ago the other molly died.
 
Now, I have just returned home after an (unexpected) 2 days away to find that 2 more tetras and 1 danio have disappeared.
 
After looking for them I found the body of 1 tetra and the poor danio, who had become entangled in the filter and looked a bit of a mess .
 
The danio was looking a little on the chubby side before it died, but the others all seem to look quite happy.
 
I'm feeding them either a scattering of flakes once a day, or some dried or frozen bloodworm as a treat (plus pellets for the plec). I also have live plants and two filters - a stingray and a EHEIM aquaball.
 
I wondered if something could be eating the tetras - but the danio death is a mystery.
 
What could be going wrong? I wanted to have 6 danio and 6 tetras so they were in schools, but now I am down to 3 danios and 3 tetras. 
 
 
I have checked my water parameters with a liquid kit and they are:
 
ammonia: 0ppm
nitrite: 0ppm
nitrate: between 0-5ppm
PH: 7.8
temp: 25 degrees
 
Thanks for any help on this.
 

 

[ncguppy830]

anything fish activivty out of the norm?
on a side note theres no such thing as a "dwarf pleco" there are smaller species though.

 

[JenLo]

Nothing out of the norm. They all look fine, and they are eating fine.
 
Ah thanks, don't know much about him, other than he is fascinating! I might put a pic up and see if anyone can tell me what he is, thanks. 

 

[ncguppy830]

hmm thats strange, i stumped, id doubt theres anything eating the tetras

 

[SmexxyNick]

Yeah ive had the tetra thing happen to me, when they die there bodys fall apart fast and just get sucked up prob,and i seen a danio of mine get really crazy bloated and then die... But haven't seen that in almost a year.

 

[JenLo]

The tanks have had a partial water change and gravel clean tonight. I just hope I don't lose anything else. 
 
Can danios explode?! It looks like that may have been what happened. 
 
Not pleasant