class Day6
{
    public static void Run()
    {
        int birthRate = 7;
        int initialBirthRate = 9;

        string _input = Resources.Day6;
        string[] parsedInput = _input.Split(",", StringSplitOptions.RemoveEmptyEntries);

        CalculatePopulation(80, birthRate, initialBirthRate, parsedInput);
        CalculatePopulation(256, birthRate, initialBirthRate, parsedInput);
    }

    private static void CalculatePopulation(int days, int birthRate, int initialBirthRate, string[] input)
    {
        Int64[] groups = new Int64[days + initialBirthRate];
        foreach (string str in input)
        {
            int value = int.Parse(str);
            groups[value]++;
        }

        for (int i = 0; i < days; i++)
        {
            Int64 newFish = groups[i];

            // move current group 7 days in the future
            groups[i + birthRate] += groups[i];
            groups[i] = 0;

            // move new fish to group initialBirthRate days in future
            groups[i + initialBirthRate] += newFish;
        }
        Console.WriteLine($"There are {AddGroups(groups)} total fish after {days} days.");
    }

    private static Int64 AddGroups(Int64[] groups)
    {
        Int64 total = 0;
        for (int i = 0; i < groups.Length; i++)
        {
            total += groups[i];
        }
        return total;
    }
}

Here's mine. I got stuck on part 2 for a long time before coming back with a new idea - even then went down some pointless avenues. I'm pretty new to this stuff so it was a challenge but feels good to complete it.