AquaIllumination HMS Multi Light TrackChoosing the right HMS Multi Light Track Add 1. 5" to your aquarium's overall length and match with the closest size that is longer than your aquarium. Example: If you have a 35. 5" tank, add 1. 5" to that length, giving you 37" total. The 37. 5" HMS track would be the correct choice. Aquariums up to 24" 25. 5" HMS Track Aquariums 25" to 36" 37. 5 HMS Track Aquariums 37" to 48" 49. 5 HMS Track Aquariums 49" to 60" 61. 5 HMS Track Aquariums 61" to 72"