Have you ever gone to check on your tank only to find that one or more of your fish have mysteriously disappeared or passed away? I'm going to run you through all the reasons this might be happening

Food Quality and Its Impact

Some other stress factors that might shorten your fish's lifespan and lead to them wasting away over time include food quality. The type of food you provide matters—if you have a fish that thrives on an omnivorous diet but is being fed a high-protein diet exclusively, it won’t live as long. Just like a human eating only Big Macs for life wouldn’t reach 100 years old, fish also need a balanced diet.

Another major factor is temperature. A very warm aquarium will cause fish to metabolize food faster, possibly become more aggressive, but ultimately have a shorter lifespan. Similarly, incorrect water parameters can stress fish. For instance, keeping a fish in very hard water when it requires soft water will limit its longevity.

Tank Mates and Aggression

Another factor to consider is your tankmates and any possible aggression. Even if you don’t see it happening during the day, fish are often active long after your lights go out. Some species, like plecos and certain catfish, are nocturnal and become more active at night, while other fish use this time to rest. If a fish is constantly being disturbed while it’s trying to sleep, stress will eventually take a toll.

The aggressiveness of fish also plays a role. For example, blue acara cichlids can be aggressive, so they should not be housed with very peaceful fish. You want fish that match in aggression levels so that any disputes are minor, like sibling squabbles, rather than constant bullying.

Additionally, consider the areas of the tank where your fish dwell. Macrobettas, for instance, are top-to-mid dwellers, whereas plecos stay at the bottom. This means they are unlikely to interact, reducing territorial disputes. On the other hand, if you overcrowd the bottom with fish, they will constantly fight over territory, leaving the upper tank looking empty. This not only makes the tank less visually appealing but also creates an unhealthy environment for your fish.

Hidden Dangers: Pests and Parasites

One of the hardest issues to identify in an aquarium is pests. Pests like planaria and hydra can prey on baby shrimp and small fish. While their impact is sometimes overhyped, their presence can reduce fish numbers over time.

Other dangerous pests include dragonfly and damselfly nymphs. These are much worse than planaria, as they are stealthy predators that can remain hidden for long periods. They often hitchhike on plants or enter tanks if a dragonfly lays eggs in the water. These nymphs can hide behind sponge filters, in plant roots, or in the back of tanks, making them difficult to detect.

Internal parasites are another major concern. It is common to see fish in both big-box pet stores and local fish stores with sunken bellies, bloating, or white stringy feces—these are signs of internal parasites. Unfortunately, these conditions can be difficult to treat. The best approach is to use multiple medications to cover different parasite types and to regularly gravel vacuum to prevent reinfection.

These are just some of the factors that might be causing your fish to disappear or die unexpectedly. Keep them in mind and investigate further if you suspect any of these issues in your aquarium.