In have been doing an NMDS of plancton, fish and aquatic macroinvertebrate abundances with 7 environmental variables in PAST. But when the analysis is complete the stress value it gives me is 0, is it normal or is there something I am not doing right?
The stress value represents how close the reduced space in NMDS is to the multidimensional space (no reduction). Thus, the higher the number of dimensions included in the solution, the lower the stress value. Technically, if you include all possible dimensions (as many as the number of variables in the ordination; 'species'), the stress value will be zero. Are you including all dimensions in your analysis?
Note that NMDS is an unconstrained ordination; thus, the 7 environmental variables you mention have nothing to do with the solution and patterns in species composition in your solution.