Betta Splenden Sudden Death

Hi guys,
 
I am hoping that someone on this forum will be able to help me understand the sudden death of my beautiful betta Nicki this morning.
 
Nicki was approximately 2 years old and was kept in a 300 ltr tank with a variety of other fish who he has cohabited with for a long time very happily.  The tank has regular water changes and all of the ammonia, nitrate, nitrite, pH, GH and KH levels were/are ok.  The tank is heated at a constant temperature using a heater and there were no sudden temperature changes. 
 
Last night we fed the tank, including Nicki, who was his usual inquisitive self at the top of the tank eating his flakes.  He seemed his normal self and was behaving as he always has done.  However this morning when we looked into the tank Nicki was rather shockingly dead.  At first we thought he was sleeping as he was lay underneath one of the leaves of a plant that we have near the bottom of the tank. But I grew suspicious when he didn't swim to the top of the tank as he normally did when he saw us.
 
All of the other fish in the tank are fine and all of the water levels are ok and I just can't seem to understand what could have caused his death.
 
Nicki is not my first betta and I am aware that they can have short life spans but surely if it was just old age there would have been some sort of sign?
 
Basically I am just hoping that someone can help to give me a little closure for the loss of my little guy.
 
Any advice would be greatly appreciated.
 
Thanks in advance.
 

When you say the tank results are all normal, what were the parameters?

What are the other tank mates?

Have you recently introduced any new tank mates?

It could totally be old age, but this just gives us an insight

We use the tetra 6 in 1 test strips and the parameters were:
NO3 = 0
NO2 = 0
GH = 150 ppm
chlorine = 0
KH = 120 - 180ppm
pH = somewhere between 6.8 - 7.2
 
These parameters are all within the "safe levels" and they've all been constant since we first got the tank and cycled it.
 
The tank mates include a bristlenose catfish, a red tail black shark, 6 tiger barbs and 6 neon tetras.  The tank mates have been living together harmoniously for around 4 months now and the last tank mates to join were the tetras about a month ago.

Do you think you could get the results with a liquid kit? These strips are not very reliable. Did your betta have any ripped fins? I know you say there hasn't been bullying, but this often happens at night. I'm thinking mostly about the barbs and the shark.

It is odd that your nitrate is zero. I've got heavily planted tanks and I still have nitrate readings.

My betta didn't have any ripped fins he was in brilliant condition (you can see a picture of him in the first post) and there wasn't any bullying going on as he was the only top dweller in the tank.  The tank is plenty big enough for them to all have there own space and there are plenty of hiding paces too at all levels in the tank. 
I don't believe the tank mates to be the issue as they had lived together with no issues for over 4 months and his behaviour didn't change after his tank move.  
 
I know for some reason that sharks have a reputation for being aggressive but our shark is really chilled out and keeps himself to himself, he has never attacked another fish in our tank.  In fact when we first got him he was in the tank with a female krib and she used to chase him despite being less than half his size.
 
I've always found the test strips to be reliable and the fact that all of the other fish in the tank are all absolutely fine again makes me think that it's not a water issue.  As for the nitrates being zero the reading is somewhere slightly above zero but not at 20 ppm both of which are in the safe zone.