Help, Fish Randomly Dyeing

[Fishyben]

Hey guys I have a 125l tank which has been set up for about 2 month and just recently my fish have been dying, the levels are;
Ph 6.5
Nitrite 0ppm
Nitrate less than 25ppm
Ammonia 0.25 ppm

I noticed on my swordtail and now my honey gourami these red spots on its body

Now my Siamese fighter male is dying and swims vertically to the top and falls back down, when on the bottom it stays on its side
Any help greatly appreciated as I'm fairly new to fish keeping
Thanks ben

 

[Fishmanic]

do a partial water change to bring the levels down--you may be experiencing a mini-cycle

 

[Ninjouzata]

Some questions that will hopefully help us figure out what's going on in your tank.
 
1. Water parameters. (ammonia, nitrate, nitrite, PH, temp', Hardness etc)
2. A full description of the fishes symptoms.
3. How often you do water changes and how much.
4. Any chemicals and treatments you add to the water.
5. What tank mates are in the tank.
6. Tank size.
7. Finally Have you recently added any new fish?

 

[Fishyben]

Chlorine is 0
General hardness is about 8
Water temp is about 24C

Doing a 25% water change once a week, recently since symptoms have started been doing it twice/3 times a week and done one today with some RO water from the LFS,
When doing a water change I generally put in tetra aqua safe, when I noticed the symptoms I treated with interpet no9 anti internal bacteria
Currently in the tank are 4 bleeding heart tetra
1 red tail black shark
6 male guppies
5 neon tetra
2 honey gourami
1 male Siamese fighter and 3females
Recently added the guppies to the tank after a few died
The fish seem to look healthy (other than the few with the red spots) then remain hidden or near the top vertical then seem to fall to the bottom and carry on going to the top falling until they die

 

[matthewkeogh50]

I think its a parasite Called Costia take a look online for treatments