Fixer can definitely be re-used.
For film I keep a 1.5 litre bottle of rapix fixer diluted to working strength (1+4, concentrate + water) with a strip of masking tape stuck across it. On the tape I record the accumulated total of the number of films fixed. When the total gets to 40 I discard the used fixer and make up a new batch. Film fixer can be worked "harder" than paper fixer because the fixation by-products (silver tetrathionate, etc) wash out of film much more easily than out of fibre base paper.