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Unfortunately, your tank is too small for a couple reasons.
One, goldfish kept in those tanks keep growing on the inside and get stunted on the outside, leading to serious health problems.
Two, they produce a LOT of waste, and with two in that tank, they are likely suffering from ammonia poisoning since the filter can't keep up.
Also, those ammo-chips eventually "fill up" and stop working...it's better to get a natural cycle going. Read the stickies on how, and please find a bigger tank.
Red gills are certainly a sign of ammonia poisoning.
20 gallon with a male betta, neons, glowlights, and red cherry shrimp. (work in progess) Recently added a few LIVE plants and driftwood, Woooohoooo!