What Is Going On! Help! 3 Dead Fish In Two Days.

[thenightgoddess]

This is not a new tank it has been set up for over a year. It's a 15 gallon tall. It has a aquaclear 50 filter. Over the past two or three days I have lost three fish and a snail. I have no idea whats going on. The first one that died I thought died because it was a new fish that I got last week and my female betta wouldnt stop chasing it. Now I have lost a male balloon belly(the new fish), my female betta, an albino cory, and a gold mystery snail. The only thing that has been wrong with the tank over the last couple months is high nitrates between 40-80 usually. I try to do a 25-50% water change every week to keep up with it. My last water change was last monday and I am planning on doing this weeks today. I just put prime in the tank in case there is an overload of nitrates. I will test the tank and add the results later after I do a water change. The tank has also been slightly cloudy the last few days. I still have a female guppy, a male variatus platy, two albino corys, and a bristlenose pleco. None of them are acting any different then they normaly do and none of the dead fish had anything wrong with them that I could see. I dont want to lose anymore fish and I have no idea what is going on this hasnt happened since I first started up the tank.
 
 
 
 
Also im pretty sure the albino cory was not dead a couple hours ago when I pulled out the female betta. There was alot of black small particles in the gravel if I didnt rinse the carbon enough before putting it in last week could that have been the problem?
 
http/i20.photobucket.com/albums/b232/hopes227/photo6_zps7ef86954.jpg
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http/i20.photobucket.com/albums/b232/hopes227/photo3_zps371d681f.jpg
 
The pictures are of the water I took out while doing a gravel change and the other three are the dead albino cory in different positions.

 

[thenightgoddess]

Just like I thought my test came out normal. PH: 7 Ammonia: 0 Nitrites: 0 Nitrates: about 30

 

[techen]

A cloudy tank could be bacteria bloom. Please post your water stats using a API masters kit or alike so we can help you out more.

 

[thenightgoddess]

A cloudy tank could be bacteria bloom. Please post your water stats using a API masters kit or alike so we can help you out more.

It has been tested

 

[thenightgoddess]

Pictures of the tank
http/i20.photobucket.com/albums/b232/hopes227/photo9_zps23c209f2.jpg
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http/i20.photobucket.com/albums/b232/hopes227/photo8_zps123e692e.jpg
Again could carbon getting into the tank be the problem? If it is what can I do about it? I got as much out as I can could when I gravel vacced and I rinsed the carbon again.