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.
