This report includes results from a number of experiments with the common purpose to identify options and solutions to promote the ongoing development of recirculating aquaculture systems. Rearing conditions and water quality issues have been the central issues in the project comprising four separate work packages (WP). The experimental approaches for all four WPs have been practically oriented towards existing management issues and challenges. The aim was to investigate and document potential important process correlations to increase the knowledge level and identify practical solutions to accelerate better management practice. WP1 “Nitrification” dealt with investigations on nitrogen removal in biofilters, in particular test and comparison of the effect of carrier medium composition on nitrite accumulation. Moving bed and fixed bed biofilter elements were compared in series of experiments conducted under similar predefined controlled conditions in a newly established RAS test facility. Total ammonium/ammonia (TAN) removal rates at 0.25 and 0.23 TAN/m2/d were found in fixed and submerged biofilters respectively; both types of filters operated under identical conditions and with equal volume and available surface area. Corresponding nitrite removal rates of 0.30 g nitrite-N/m2/d was found in fixed, submerged biofilters compared to 0.23 g nitrite -N/m2/d I moving bed biofilters. Experiments with sudden changes in N-loading caused proportional elevation in TAN levels and led to significant prolonged nitrite accumulation with concentrations up to 6 mg nitrite-N/L. Active biofilter elements were transferred to reactor-tubes and exposed to TAN or nitrite spiking, revealing that fixed bed bio-elements were more heterogeneous than moving bed bio-elements. Depending on the vertical positions of the bio-elements from the fixed biolfilter, TAN and nitrite removal rates ranged from 0.05 to 0.35 g N/m2/d compared to moving bed elements with measured removal rates around 0.17 – 0.20 g N/m2/d. This work package also included diurnal investigations on Model trout farms where TAN, nitrite and nitrate concentrations were measured. The studies showed a marked variation in TAN levels between systems ranging from less than 1 g TAN/l to more than 15 g TAN/l. The studies documented that corresponding actions in terms of increased aeration and alkalinity adjustment increased nitrification rates allowing rapid TAN concentration reduction in the particular RAS