Trouble With A Glolight Tetra

I have a small Desktop sized tank with three Glolight Tetra and an Upside-Down Catfish. I've had them for 7 months and have noticed that one of the Tetra keeps getting fatter and fatter while the rest remain sleek. The fish doesn't seem to have anything wrong, in that the scales are fine and there's no troubles with eating, swimming, or anything else. My trouble is the fish is usually really feisty and has earned the name Lucifer in return because it likes to chase the others, and as of late it has been more laid back and continually getting fatter. As I said the scales and fins look fine, everything else seems normal, I'm just worried because I don't want to lose one of my fish. Any help possible is much appreciated!!

Is it possible for you to get a picture of the fish? It's really impossible for us to tell what might be wrong, just from your description.

If you could also tell us how big the tank is, exactly (measurements, or gallons or litres; any will do), what sort of filtration you have, how often and how large your water changes are, and the results of any water tests you might have done, that will all help too

It's a 1.5 Gallon tank with a Whisper filter. I change out half the water every weekend and use Tetra Balance, both Weekly and Monthly Maintenance. All the water tests show healthy levels and it's even been confirmed by taking samples to a petshop near me. I tried asking them, but they can't tell me what's wrong and unfortunately the best pic I can get is a little blurred but you can see the difference in size between Lucifer and one of my other fish. Lucifer is the Yellow one on the left.

1.5 is wayy to small to house any fish except like snails. they are prolly suffering from ammonia build up. ps dont use balancing stuff. 

I was told with how often I change the water that it would be fine. I've also used the Tetra Balance in four other tanks in the past and had no troubles. This is my first ever instance where I've had a fish with this problem. Granted previous tanks were much bigger and kept on stands proper due to the size, but this was a present that was supposed to help with a handful of other things since fish tanks have calming qualities to them. I don't want to hurt my fish, but at the same time I currently don't have the money or space for a bigger tank and as was stated I've had them for 7 months and have had no troubles until now.

toomanytroubles said:

I was told with how often I change the water that it would be fine. I've also used the Tetra Balance in four other tanks in the past and had no troubles. This is my first ever instance where I've had a fish with this problem. Granted previous tanks were much bigger and kept on stands proper due to the size, but this was a present that was supposed to help with a handful of other things since fish tanks have calming qualities to them. I don't want to hurt my fish, but at the same time I currently don't have the money or space for a bigger tank and as was stated I've had them for 7 months and have had no troubles until now.

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Pet stores will tell you anything. I promise this is not ok. How many fish have you had die in there?

So far? None, and I bought my fish from a specialty store. However number of fish dying is not my problem, I would like help in finding out if I need to be concerned about my Yellow Glolight Tetra or if I just happened to get an immature female and didn't know it.

The cream-coloured fish in the photo is not a Glolight Tetra but a Glo Fish.  This is a man-made form of the Zebra Danio.  Many aquarists are opposed to these practices of genetic manipulation and won't buy the fish.  Regardless of that, I'm afraid a 1.5 g is way too small for a group of this species, and they are shoaling fish that should be in a group of six or more but in at least a 20g long.
 
I agree with Ryan that a 1.5 g tank is not sufficient space for fish.  I know pet stores (and some responsible ones too) will promote such tanks and have small fish in them, but that does not make it the best environment for the fish.  A 5 gallon is about the smallest you want to have for fish, and then you would be restricted to some of the very small cyprinids perhaps, or a single Betta.
 
I also agree on the additives; any chemical or "biological" substance that claims to make things better is probably doing much the opposite, for reasons I haven't the strength to go into now but may later.  Your water changes are much better remedies, keep them up regularly.
 
The fact that a few fish can manage in less than ideal conditions does not mean they are not being affected.  I believe they are.  This species as one example needs more of its own to interact, and being an active fish it needs space to swim fast; that is how it has been programmed by natural selective evolution, and when it is forced into different environments it is stressed, and over time this takes its toll; see both citations in my signature.
 
When testing water, always have numbers.  Ammonia, nitrite and nitrate are important here, as is pH.  "Healthy levels" may mean something very different to the staff in the store, or you, or me.  Healthy levels for ammonia and nitrite are zero, nothing else; nitrate should be low, no higher than 20 ppm.  The pH can vary depending upon the source water, water changes, tank conditions, etc.
 
Byron.

P.S Bryon, I like your sig, it applies to this situation well