There is no need to have separate pots for fish and meat. While there is a danger to mix fish and meat together, PlaitasKelim, the taste that remains in pots, is not a danger, and the same pot may be used for both fish and meat. While Yesh Machmirim, there are those who are strict to designate separate Kelim, separate pots, for fish and meat, this is only a Chumra, a stringency, and only lechatchila, in the first instance. After the fact, if fish was cooked in a meat pot or vice versa, all agree that there is really no problem. A possible explanation for this Chumra is that often it is difficult to clean a pot thoroughly, and it is possible for some fish or meat remnants to have remained in the pot. However, if the pot is absolutely clean, it seems like there would not be any problem to use the same pot for fish and subsequently for meat or the opposite.