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If your goldfish has been spending his time at the bottom of the tank sucking in gravel and spitting it out, don't panic. He's either scanning for a quick snack, or possibly smitten with a female tankmate.
All You Can Eat
Goldfish are opportunistic feeders: If they can eat it, they will try. Part of this ongoing buffet is the tempting bits of algae and microscopic organisms that live on gravel. Because their fins aren't exactly equipped to pick off these tasty morsels, goldfish will pick up a pebble, scrape off as much algae from the gravel as they can, then spit the gravel piece back out.
Sometimes, during this sifting and scraping and sucking procedure, bits of the gravel will be included in what the goldfish consumes. There's usually nothing to worry about if this happens; these small rocky bits will pass through the fish's digestive system and be excreted in his waste. If you see tiny pieces of gravel in the feces of your goldfish, you'll know exactly what he's been up to, and that it's normal.
Another possibility is that your finned friend has found romance. When it's mating season, both male and female goldfish will begin to nip at anything they can get their mouths on -- plants, gravel and especially one another.
This nipping is natural, but it is a good idea to keep an eye on the two lovers to make sure one is not being too aggressive towards the other. If you prefer your goldfish to not mate due to this behavior, it would be best to separate Romeo and Juliet into different tanks.
To avoid situations such as stuck gravel, make sure the rock size of your substrate is larger than pea gravel. Another option is to use fine sand to line the bottom of your goldfish's aquarium. Before selecting a sand, research it to make sure it will not irritate the gills of your fish. Not all sands are created equal, and some sands can trap toxins, cloud the water or ruin water pumps with debris. A good-quality fine sand, however, will help keep your fish safe no matter how often he gets the munchies, or falls in love.