Perfect Conditions, Dead Fish

[Zoey8778]

Last night I changed 20% of my water in my 10 gallon tank I had 3 tetras 3 swordtails and 1 male betta living a mesh breeder trap because I could not maintain his water in the his 3 gallon bowl. This morning one of the tetras was dead and  floating near the betta's trap. Now the is swimming on its side and looks a liitle sick, he seemed very happy in the net. So I moved him back to the bowl with water from the 10 gallon and some new water. I always did 20% changes in the past. 

 

[Bubble.Guppy]

What do you mean by perfect conditions? Do you use a water kit like API?
And the other fish looks a little sick? Anything visual?

 

[walklong]

Last night I changed 20% of my water in my 10 gallon tank I had 3 tetras 3 swordtails and 1 male betta living a mesh breeder trap because I could not maintain his water in the his 3 gallon bowl. This morning one of the tetras was dead and  floating near the betta's trap. Now the is swimming on its side and looks a liitle sick, he seemed very happy in the net. So I moved him back to the bowl with water from the 10 gallon and some new water. I always did 20% changes in the past. 

What are you using to condition (de-chlorinate) your change water? Did you mistakenly rinse filter media in straight, chlorinated tap water? What kind of tetra? Some are quite sensitive to pH & temperature differences. 

 

[Ch4rlie]

A few suggestions above are noteowrthy but the main question I think we should be asking is -
 
What are your water parameters first of all?
Ammonia, nitrite and nitrate
 
 
Is your tank cycled?
If unsure what i mean, have a look at this link - http/www.fishforums.net/index.php?/topic/421488-cycling-your-new-fresh-water-tank-read-this-first/
 
 
What is your usual weekly routine in terms of water changes and tank/filter maintenance?
You say you've always done 20% changes in the past, this tells us little. If you mean 20% once a week, then this is not quite enough to deal with the bioload and mess that your swordtails and tankmates makes in a little 10 gal tank for a start.
 
I do hope you are aware that 10 gal tank is not suitable for swordtails, tetras and a betta. As ideally should just be a betta or perhaps  just a small group of schooling fish such as ember tetras, but am unsure which specie of tetras you have in your tank currently.
 
A few questions there I know, but answering these will help us get a general idea of what is happening with your tank.

 

[Zoey8778]

Last night I changed 20% of my water in my 10 gallon tank I had 3 tetras 3 swordtails and 1 male betta living a mesh breeder trap because I could not maintain his water in the his 3 gallon bowl. This morning one of the tetras was dead and  floating near the betta's trap. Now the is swimming on its side and looks a liitle sick, he seemed very happy in the net. So I moved him back to the bowl with water from the 10 gallon and some new water. I always did 20% changes in the past.

Update: yes, the water changes are weekly, the tetras are black skirt, I use a API test kit, is it possible the tetra got into the betas cage and they just fought it out. The net had shifted overnight leaving a small swim space for both to swim over. I use a jacks pets brand de-chlorinator, that is a small pet store chain that specializes in aquarium care. Would a 20 tall fit my 10 gallon stand, and I did not do anything with my media during this change. The other fish seem to going about business a usual and appear heathy, the tank is probably 5 months old now and is fully cycled.

 

[Bubble.Guppy]

 

Last night I changed 20% of my water in my 10 gallon tank I had 3 tetras 3 swordtails and 1 male betta living a mesh breeder trap because I could not maintain his water in the his 3 gallon bowl. This morning one of the tetras was dead and  floating near the betta's trap. Now the is swimming on its side and looks a liitle sick, he seemed very happy in the net. So I moved him back to the bowl with water from the 10 gallon and some new water. I always did 20% changes in the past.

Update: yes, the water changes are weekly, the tetras are black skirt, I use a API test kit, is it possible the tetra got into the betas cage and they just fought it out. The net had shifted overnight leaving a small swim space for both to swim over. I use a jacks pets brand de-chlorinator, that is a small pet store chain that specializes in aquarium care. Would a 20 tall fit my 10 gallon stand, and I did not do anything with my media during this change. The other fish seem to going about business a usual and appear heathy, the tank is probably 5 months old now and is fully cycled.
 

 
What are your water parameters from your API Master kit testing?