The Comet raised flags immediately. I looked up the ingredients and two struck out immediately, fragrance and coloring. There are a couple of the individual chemical components that could be responsible as well, but I don't know how the configuration of the material is and they may be neutralized by their configuration in the compound.
As for fragrances and coloring. With fragrances you really have no way of determining exactly what chemicals are in them. The chemical formulas for fragrances are rarely available even on-line. Anything in them may be harmful or deadly to fish, and it may be released through interaction with another chemical, such as CO2 or a dechlorinator. You can often find the chemical formulas for coloring, but they are often quite complex and it's next to impossible as a hobbyist to determine what chemical reactions may take place.
I can't say 100% what the problem was. You said you're tetras have been living in the tank for years. My gut feeling tells me that there may have been some mild toxicity due to the residue left from the Comet, and some of your tetras were old enough that they couldn't withstand the stress. Like I said, I can't say 100%, but considering everything you've provided I'm wondering if it's something along these lines.
There are only four things I use for tank maintenance. Plain baking soda if I need a mild abrasive or need an absorbent. Pure white vinegar (no flavored vinegars), which is organic and breaks down quickly if there is any residue. Pure chlorine as any residue can be neutralized by a dechlorinator. And pure ammonia (not scented) as residual will be nitrified by the bacteria in the filter. You really have to be careful with ammonia and make sure it is rinsed off thoroughly. I usually only use ammonia on equipment before it is installed and before critters and plants have been added, basically pre-cycling only.
WYite
As for fragrances and coloring. With fragrances you really have no way of determining exactly what chemicals are in them. The chemical formulas for fragrances are rarely available even on-line. Anything in them may be harmful or deadly to fish, and it may be released through interaction with another chemical, such as CO2 or a dechlorinator. You can often find the chemical formulas for coloring, but they are often quite complex and it's next to impossible as a hobbyist to determine what chemical reactions may take place.
I can't say 100% what the problem was. You said you're tetras have been living in the tank for years. My gut feeling tells me that there may have been some mild toxicity due to the residue left from the Comet, and some of your tetras were old enough that they couldn't withstand the stress. Like I said, I can't say 100%, but considering everything you've provided I'm wondering if it's something along these lines.
There are only four things I use for tank maintenance. Plain baking soda if I need a mild abrasive or need an absorbent. Pure white vinegar (no flavored vinegars), which is organic and breaks down quickly if there is any residue. Pure chlorine as any residue can be neutralized by a dechlorinator. And pure ammonia (not scented) as residual will be nitrified by the bacteria in the filter. You really have to be careful with ammonia and make sure it is rinsed off thoroughly. I usually only use ammonia on equipment before it is installed and before critters and plants have been added, basically pre-cycling only.
WYite
