Sick betta fish, help!

[Silvercloud]

I have been watching the bettas in my tank for about three days now. One of them is acting extremely lethargic, not moving and laying on her side. The other betta is also very lethargic, and she is swimming vertical most of the time. There are two more fish in the tank who are doing just fine. Please help, I lost another betta from the same tank the same way, and cleaned the entire thing. Any guess is greatly appreciated, thank you.

P.S., I have already fed them a pea, it didn't work

 

[ZxC]

Ammonia?
Nitrite?
Nitrate?
pH?
Water temperature?
Tank size?
Tank age?
Filter make and model?
Water change schedule?
Cleaned "the entire thing" with what and how?
What are the other fish in the tank?

 

[Silvercloud]

My current test kit doesn't tell ammonia, I definitely will check on one that does
Nitrite is 0ppm
Nitrate is around 20ppm
pH is 8.4, roughly
Water temp. Is 66, which is too low, working on buying a better heater
Tank is 5 gallons
Roughly 1 year old,
Aqua tech 5-15 gallon filter
Water changes every Saturday
Just scrubbed the entire thing with a sponge until it was spotless, changed all the water, the filter, and the heater ( before I set the tank up again around a year ago)

 

[Silvercloud]

Also the other fish are an ender guppy and an albino corey cat

 

[ZxC]

My current test kit doesn't tell ammonia, I definitely will check on one that does
Nitrite is 0ppm
Nitrate is around 20ppm
pH is 8.4, roughly
Water temp. Is 66, which is too low, working on buying a better heater
Tank is 5 gallons
Roughly 1 year old,
Aqua tech 5-15 gallon filter
Water changes every Saturday
Just scrubbed the entire thing with a sponge until it was spotless, changed all the water, the filter, and the heater ( before I set the tank up again around a year ago)

Okay, so the tank is cycled that's good. Temp is way too low for bettas, probably part of the issue. What other two fish are in there? pH is also way high, perhaps look for distilled or Reverse Osmosis water from a supermarket? On a 5 gallon with 25% water changes that isn't terribly hard on the wallet. Mix say 50/50 tap water to RO water. Are you treating the new water with dechlorinator?

 

[Silvercloud]

I am using dechlorinator on the new water, other fish are an endler guppy and an albino corey cat

 

[Angellove78]

You gotta do something fast about that heat... way, way, way too low. If you have a heating pad put that around the tank or closer to a heater at least. It should be 78-80 degrees. Put a towel around it. Something like that.