2 Dead Fish In Under A Week Betta

[mobo]

I have a tropical tank for about a over a month

Tank size:
nano cube that is 48cm wide 105 litre 

tank temp: 24c

Fish Symptoms (include full description including lesion, color, location, fish behavior):
yellow platy added to tank on friday dead on sunday.
Siamese fighting fish added to tank 4 weeks ago seemed perfectly happy and active untill today when we couldn't find it in the tank. It turned out to be hiding behind the heater and when it moved wasn't swimming. We put fish in separate tank with some medicine and it had been on its side all day and died tonight

Volume and Frequency of water changes: 20% 2 times per week

Chemical Additives or Media in your tank:

Tank inhabitants: 6 neon tetras, 4 guppies, 3 platys, 3 zebra danios

Recent additions to your tank (living or decoration):
july 1st 6 neon tetras
july 10th 1 Siamese fighting fish (dead)
july 17th 4 guppies
august 2nd 4platys (1dead) 3 zebra danios
 
Exposure to chemicals: clorine cleaning chemical

Digital photo (include if possible):

 

[Chad]

Is it possible that the chlorine cleaning chemical actually got into the tank?
 
Of course chlorine is an antibacterial and could potentially kill off all the beneficial bacteria in your tank. This bacteria is what gets rid of the harmful ammonia and nitrite.

 

[Blondielovesfish]

Is the tank cycled? What are the ammonia and nitrite readings?

 

[SimpleA]

As above, definitely need to check those readings.  If these are wrong fish can go very quickly.  Either that or a bad batch of stock?

 

[peds]

Siameses fighting dont like big tanks. Better keep them in small ones instead as they get "depressed". They will start hiding themselves all the time, loosing their fins till they die, Unfortunately.

 

[Wildbetta]

Siameses fighting dont like big tanks. Better keep them in small ones instead as they get "depressed". They will start hiding themselves all the time, loosing their fins till they die, Unfortunately.

 
This is a horribly wrong statement!  Bettas (I HATE the term Siamese Fighting Fish) love space and do best in bigger spaces.  You do have to make sure that you have enough plants and decor so that the betta has enough places to hide and feel comfortable though.  How many gallons/liters is your tank?  My thoughts are that the size is probably a bit small for all the fish you added.  Bettas are also not really good as community fish and either get aggressive or highly stressed when in the presence of other fish which normally leads to them dying. (this is if the betta is not one of the very few that actually thrives in the community setting)   

 

[mobo]

I need to order a kit to test the ammonia and nitrate
the tank hasn't been cycled
it is 105 litres. it seemed to like the environment as there was plenty of bubbles on the plants and around the top of the tank#
I used the community creator on think fish and it says i have only used 63% of the tank space

 

[Valiant]

The most likely problem is the water qualitiy is bad or you haven't cycled it.

 

[Blondielovesfish]

The fact that the tank isn't cycled is your problem.
Until you receive your ammonia test kit, do daily water changes of AT LEAST 50% daily. Remember to use dechlorinater and temperature match the water.