The thing I don't like about Tetra is their ingredient splitting - I notice they use a ton of starch and split just about everything so it looks as though fish is the first ingredient in the food. Here are the first few ingredients:
TetraFin Goldfish Crisps (Tetra Holding Inc, tetrafish.com)
Ingredients:
fish meal
fish oil
dried yeast
corn starch
shrimp meal
wheat gluten
ground brown rice
potato protein
dehulled soybean meal
monobasic calcium phosphate
algae meal
lecithin
krill
Notice that yeast, corn starch, wheat gluten, brown rice, soybean meal are all up there with the fish meal - by percentage, the food is mostly starch, not mostly fish-derived ingredients. I think you might get more fish in cat food!
Also, they use red, yellow and blue dyes, many of which have been found to be carcinogenic.
The above quote came from this thread, which I found extremely useful. They break down most commercial foods by ingredient, percentage, vitamins...
http://thegab.org/forum/viewtopic.php?t=3662&postdays=0&postorder=asc&start=0
I do wish these were compiled into tables to make them easier to read, though.
I'm not sure what you mean by the "preservatives" in OmegaOne, reptile. They use ethoxyquin like nearly all commercial foods, including New Life Spectrum. In fact, the FDA requires it to be added to all fish meal, unless the company applies for a special permit to exclude it. I'm not sure about the "natural and artificial colors" in OmegaOne, but I suspect that is probably astaxanthin, derived from krill and spirulina, which is added to many other commercial foods, even some gel foods.
I'd love to try the Mazuri gel foods, but I don't have a blender right now. It would be so nice to control exactly what I put in their food, not just supplement their diet. Even the best commercial foods are still... well, commercial and highly processed.