The aim with this study was to investigate effect of various RAS water treatment designs on water and biofilm microbiota, survival, growth and gill health of lumpfish. An experiment with lumpfish was conducted, from 2 months post hatch to the transfer into sea cages. Five different water treatment regimens were compared: 1. RAS with no additional water treatment , 2. RAS with a filtration unit for removal of small particles, 3. RAS with filtration and disinfection with UV-irradiation, 4. RAS with filtration and disinfection with UV-irradiation and ozone and 5. a reference FTS. The microbiota of the rearing water and tank-wall biofilm were sampled and characterized by Illumina sequencing of 16S rDNA amplicons.