I have a 20 gallon, tropical, freshwater aquarium that has cycled for three years. However, my aquarium has unexpectedly developed issues that I am unsure how to solve.
The problem started after I bought a Harlequin rasbora from the local aquarium store. I assessed the fish to assure that it had good health. Satisfied with its behavior, I took the fish home and started my acclimation protocol. I acclimated my fish to 75 degrees Fahrenheit by immersing the store bag in my aquarium. Every 10 minutes, I added one tablespoon of aquarium water into the bag as to acclimate my fish to the aquarium's chemistry. After 30 minutes of acclimation I poured most of the bag water down the drain. Only my fish and one tablespoon of bag water entered the aquarium. The new fish showed no signs of distress and schooled with my other Harlequin rasboras. Then, two days later, it died without any signs of injury.
Since then, my fishes have successively died from “unknown" causes. Each fish swam and ate normally before death. No fish displayed signs of illness or injury - except for a skirt tetra with excessive fin rot. I doubt that infection caused the problem though. I treated my aquarium fishes with API Melafix before adding the new Harlequin rasbora. An overdose of melafix seems unlikely as well, since*I performed two 30% water changes (one week apart) after the five-day medicine treatment. Also, I reinserted the carbon filtration to ensure that no melafix remained in my tank.
After three more fish died, I decided to retest my water chemistry parameters one morning. I used two methods for assessing my tank's water chemistry. I used an API Test Strip kit to measure the pH and hardness inside my aquarium. Then I used an API Freshwater Test kit to measure the pH and nitrogen waste levels. Both kits indicated that pH, hardness, NO2
, and NH3
levels were within acceptable parameters for tropical aquarium fishes. Nonetheless, another fish died later that evening.
I urgently need help solving this issue. Fish that die without any reasonable cause does not occur. However, without any signs of illness and great water chemistry, I am at a loss of how to identify the mysterious “killer”. Any other ideas of what I can do to prevent further deaths?