The population of two fish species is being closely monitored. It is observed that the population of “species A” grows at a rate of 20% per month, while “species B” grows at 15% per month. When the population of “species A” was found to be 400 fish, the population of “species B” was recorded as 450 fish. Determine a suitable formula to model each population, and hence identify which species will first reach 1000 fish.