i have a fish only system that gets neglected and it hits 160ppm nitrates at times ! doesnt hurt any of them but it is full of tough fieh such as morey eels and some formosa wrasses. they have a ALL been in this water for at least one and some for four years .i would not recomend to do this on purpose but i seriously have to many tanks and spend my time on my reef tanks,they are in beautiful condition. in saying that i do not think that you should add anything ,let nature take its course . if i was to add anything maybe anitrifying bacteria type additive say nitro max or cycle. good luck