A fellow hobbyist once told that when a fish dies in his tank, he just leaves it there to rot because “that’s what happens in nature.” While he was right about the decomposition of a dead fish being natural, allowing it to happen within the confines of a closed aquarium system can be perilous depending on a variety of factors. Thus, the usual hobby admonition is to remove any dead fish as quickly as possible to avoid polluting the water.
But let’s face it, sometimes finding and retrieving a dead specimen from an aquarium can be fraught with difficulty if not downright impossible. So is it ever okay to just let a dead fish lie, or is it always imperative to conduct a search-and-recovery mission?
While I advise removing a deceased specimen whenever possible/practical, it might be better to just leave the body in place if:
You have a big tank and a small fish
A small, slender-bodied fish decomposing in a large aquarium will have minimal impact on water quality—especially if you have plenty of biofiltration in the form of live rock to keep pace with the ammonia produced. What’s more, in well-established systems, the resident assemblage of scavenging/decomposing animals will usually make quick work of a small dead specimen. So, if retrieval is impractical in a larger tank, there’s usually no need to fret.
However, keep in mind that even a relatively small decomposing specimen can quickly foul the water in a nano tank—ditto a larger, robust-bodied fish in a medium to large system. So, in these circumstances, it may well be worth making a reasonable effort at retrieval. At the very least, be prepared to perform copious water changes to dilute the resultant ammonia.
Recovery would be too disruptive
Oftentimes, fish have a bad habit of secreting themselves away in some tiny recess in the rockwork just before dying. If accessing (or simply locating) the body means tearing apart all the rockwork that serves as a foundation for corals and/or is held together by a thick layer of coralline algae, then you may be better off leaving the body to the scavengers/decomposers and stepping up your water changes.
Your tank isn’t on the verge of biological overload
Just as adding one more fish to an already overstocked, overfed, and/or under-maintained aquarium can overwhelm the biological filter and lead to an ammonia spike, leaving a dead fish in such a system can prove to be the proverbial straw that broke the biolfilter’s back. So, if your tank is already flirting with biological overload, a little extra effort to recover a dead specimen may be in order—not to mention, you should probably take some steps to reduce the amount of still-living livestock in the system to help bring things back in balance.