Nitrates and phosphates will probably accumulate. Neither one really seems to bother fish too much, but I know nitrates can block iodine uptake.
Folks usually use nutrient levels as a bell-weather for when water changes are needed. Nitrates <10 ppm is one rule of thumb, PO4 >.05 is another....but they are just rules of thumb.
IMO, it behooves you to try and maintain reef-quality water for fish too.