I'm sorry you lost your little guy. SIP Orange. :-(

While it's hard to say for certain what may have lead to his death, there are a couple of things I can point out that may have had an impact.

Firstly - you weren't doing enough water changes. For a 2gal tank - he would need 2 50% water changes WEEKLY for a CYCLED tank. And by cycled I don't mean filtered - I mean a tank that has gone through the biological process known as the nitrogen cycle. We can't know if your tank has or not, as we don't have readings for ammonia, nitrites, and nitrates. An UNCYCLED tank should get 1 50% and 1 100% water change per week.

At 2 months in - if you were only changing water 1/3 per week, there are a couple of things that could have happened directly related to water quality. Firstly - he could have gotten ammonia poisoning and died from a buildup of ammonia in the tank. Secondly - your tank could have started to cycle on his own which could have lead to a rise of NITRITES in the tank - which would have lead to NITRITE poisoning. 2 Months actually fits this scenario pretty well.

Beyond that - you mentioned you had a suspicion with the heater but you didn't specify what. If the tank were to heat up too much - that could certainly lead to a fish's death. If the heater stopped working entirely - it could have stressed his immune system to make him susceptible to other issues...and could have played in with there as well.

What I would recommend in the future is to 1. Increase your water change frequency and 2. Get an in-tank thermometer to keep tabs on temperature. The increased water changes will help prevent any issues with water quality in the future, and the thermometer will help you determine if your heater is working properly.

If you can swing it - a test kit to test for ammonia, nitrites and nitrates would also be recommended. This can give you a good handle on what your tank water parameters are at any given time, which is really important for your fish. The API Freshwater Master Kit has all 3 tests - and is cheaper in the long run than test strips.

 

You don't always see ammonia burns or fin rot on fish that are exposed to ammonia. Especially if they are exposed to very low amounts but for a long period of time. I've had fish that were swimming around in over 6ppm of ammonia, and apart from some skittishness, there were no other symptoms.

I would guess that poor water quality may have been a contributing factor to the death of your fish. Without a test kit though, it is impossible for us to know. I highly recommend if you get a new fish, at least pick up a test kit for ammonia. I'm from Oz and I've used the API branded test kit in the past, and currently use an Aquasonic branded one.

Also having a test kit, will give you a guide as to the quality of your tap water. No use doing more frequent water changes, if the water coming out of your tap is toxic.

At this point, my suspicion is that perhaps your fish was exposed to low levels of nitrites/ammonia over a long period of time and that this has weakened his immune system and eventually he has passed.

Either that, or your heater has been making the temperature fluctuate and this has had the same effect as what I mentioned above.

 

Sorry Greenapp1es I accidentally forgot to put in the causes that I think might have happened. I have updated it now.
Thanks for your help. I limited the water changes because I didn't have a gravel cleaner and had to scoop out the water manually. I may get a gravel cleaner to make it easier to change the water. On that note can anyone recommend some good gravel cleaners that are cheapish and good? An Aqua One gravel vac is available at my local pet store. Another thing is that he was sort of lying against the back of the tank in an upright position with the plant holding him up when he passed away. Very odd.

 

I've only ever seen Aqua One siphons available at fish/pet stores.

Here are some options available from an online store I frequently use.

http://www.thetechden.com.au/SearchResults.asp?Search=gravel+siphon

 

Hey there, I come from Australia too I suspect we even have the same tank haha (the 9L betta home + contents set?). Although I've since tossed the filter for a sponge filter, my heater works well and is monitored with one of those external thermometer strip ($5).

I got my gravel cleaner from city farmer (now called petbarn) for $9.96, not sure what brand, and it works completely fine. Although I only use it one out of two of my water change schedule - the 50% change on Sunday. Tuesday/Wednesday I just use a cup to do a 25% change. Hope you have better success on your next betta!

Something else to think about - did you change your filter cartridge? Theoretically it may be harmful if you leave carbon in for longer than it should be. I have heard different things about this though - some say it doesnt affect the tank water, others claims alters its chemistry. Too lazy to do in depth research, so I got rid of it. (also since each filter was $5...)

 

You can use a piece of airline tubing as a siphon/vacuum. That's what I use and it works great in smaller tanks.

 

I use a turkey baster to clean my water but I have sand substrate. I hate the siphon...

 

ZeroChan yes that is the tank I have. Also I have been strongly advised on not to change the cartridge/sponge as that is where most of the bacteria lives. (think of a kitchen sponge) I do need to get a thermometer as well. I have an old Marina heater (those plastic covered 'unbreakable' ones) from my old tank so I was thinking I should try that instead (but its really dirty and grubby so it will need a good scrub!)

 

The sponge doesnt need to be changed, but carbon does. Like I said, not too sure about what happens if you leave old carbon in tank. It is probably okay but is something to consider (have a read about carbon filtration in aquarium: http://www.ratemyfishtank.com/articles/106 it is quite interesting). The cartridge filter that comes with the tank set uses a carbon-based cartridge.
Although this is unlikely to be the cause of your fish passing anyway. Just a bit of extra info.