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I have two danios.  One appears healthy and active and is about 2 years old.  The other is over 4 years old and is spending more and more time on the bottom and looks like it is losing weight, and is very lethargic.  Is this what "old age" looks like for danios?  Is there anything I could try to get that older fish to perk up, or is it on its way out do to "old age."  I changed the filter after I noticed the decreased activity level of the less active fish.

If the water conditions are in proper condition, it is very likely that the older danio is just reaching it age limits.  You might try putting one tablespoon per 5 gals of water of aquarium salt into the tank.  Do NOT use table salt - get salt for the aquarium at your local fish or pet store.  This might help, but my guess is that it is just reaching old age.