Generally speaking, what came out of your filter is nothing compared to what comes out of mine when I clean it. But I don't clean it as often and I don't get any spikes.
The only difference is that I find mulm in the filter and it isn't grainy at all. Are you sure your filter isn't sucking up sand? If you sand is too fine maybe it goes in the wrong places, decreasing performance too. It's just a guess. When plants starts to melt away, just cut the melts as soon as you see them but it still shouldn't be enough to cause a spike like that.
Sorry about the babies. I don't think they can handle any ammonia/nitrIte very well. Keep doing what you are doing and if not this time, it will be fine next time.