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.