The Museo de La Salle–Bogotá (MLS) houses biological specimens of which 30% (17,001 specimens) are stored in fluid (ethanol 70%). The collections have historical significance for documenting Colombian biodiversity and contain the oldest zoological specimens in the country. For these reasons, the curation and management of the specimens require special attention and endless vigilance. In this study we assessed the decrease in ethanol level in jars of the MLS fluid collections over a 3-year period from 2013 to 2016 and evaluated the relation between container and lid type and ethanol loss. We did not observe a relevant decrease in the percentage of ethanol levels during 2013–2016. However, the type of lid used influenced the amount of ethanol that evaporated; we found that the pressure and twist lids were more effective in maintaining the ethanol levels in the containers, since they have a pressurized rotation system that allows a more secure closure in contrast to the pressure cap, which does not prevent the evaporation of ethanol. Based on our evaluation, we suggest the use of pressure and twist closure caps or caps with a continuous thread closure type along with a tamper-evident seal between the jar mouth and the container cap to ensure a tight and secure seal for minimizing the risk of fluid loss and ensuring the preservation of fluid-fixed specimens of this historical collection.