Basic rule of thumb that I've kinda been going by is 1" a month. with my orino that I used to have and would only eat talapia slices only grew about a 1/2"-1" a month once it was on pellets which took for ever to get him to take then it stuck at 1" a month. Monos I don't have any experience with but ppl say they are quick growers and the tems I have had since July and they were 1.5"-2" and now are 10"-11" so about 3" a month in growth but it depends on spiecies . Otherwise I figure a 1" a month is a good growing rate but some do grow faster than others