Fish dying one by one- no signs of sickness

Well first one of my cories died. Then 1 month later, my betta Candy, who was in the same tank, also died. This is two weeks from then, I have a baby blue acara in the tank, and I plan to make it a cichlid tank. Well two days ago, two weeks since Candy died, my sis found a dead black neon tetra and told me. I removed it, and today my bro found another dead black neon tetra. I immediately tested parameters, they are:
Ammonia 0ppm
Nitrite 0ppm
Nitrate 5ppm
Ph 7.2
All fine.
yesterday I took out all plants because the cories had kicked up a lot of dirt from the sand and it was flying all over the tank and landing on the plants. The plants are all in the breeding tank. Water changes yesterday as usual. 70%. For some reason the water is cloudy. Only thing I have recently added is filter media. I usually only have filter floss, this is the first time in all my fishkeeping hobby that I’ve added filter media.
Looking at my tank now, it’s not actually cloudy. It just has a lot of bits flying around in it. Luckily, I added filter floss and it’s catching the bits.

 

What filter media did you add?
What size is the tank?
Temperature?
Do you have a very reliable thermometer to monitor the temp? I ask because a 70% water change changes A LOT of things. And if you are not carefully monitoring the new water, the temperature is going to fluctuate too. and it might be too much for some fish.

 

What filter media did you add?
What size is the tank?
Temperature?
Do you have a very reliable thermometer to monitor the temp? I ask because a 70% water change changes A LOT of things. And if you are not carefully monitoring the new water, the temperature is going to fluctuate too. and it might be too much for some fish.

I added this filter media.
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The temp is 25. I have a unreliable thermometer but I use my hands and the temp was fine. I always use my hand to stop the flow of the hose from kicking up sand, and the temp is always the same.
And I know the temp is 25 because I used one of my moms new cooking thermometers, and they are very accurate.
and the size of the tank is a 55 gallon

 

Well, it could be bad breeding/badstock. Somtimes you get a string of week fish.

 

Well, it could be bad breeding/badstock. Somtimes you get a string of week fish.

Ohhh ok. Thanks for the help, it is much appreciated

 

dead rummy nose tetra 2 days ago, belly up cardinal and a cory at the surface. I also have biofilm. What could be wrong? Shall I move the fish to breeding tanks?
Well, was taking all the fish out and moving them to breeding tanks when I noticed that the Cory at the surface had pop eye. My brother was helping me catch the fish and he said it’s cruel for him to suffer like that, he’s gonna die anyway. I agreed, soo he took it out of the tank and put it so it would die quickly and it’s pain would end. Then he buried it, but I couldn’t look. Thank goodness I have such a nice brother.
So I moved all the tetras to the 12g with a sponge filter, and the baby acara to a 15g with a diy sponge filter I made by wrapping filter floss round an air stone.

tank that tetras are in


tank that acara is in.