I converted from tubes to a fluval fresh and plant
Hello; First is I am not familiar with what "tubes" are. I sort of know about fluval because I use their root tabs for my amazon sword plants. So some more information might help.
About the water sprite I have had the same experience several times and I think I can tie in a theory with the java fern. Most plants can take nutrients directly from the water but some such as the swords do not so much. Water sprite I believe does take nutrients directly from the water.
I have had it grow fast when floating in the water and not rooted. My thinking is the water sprite starts out strong and then slows in growth as the nutrients are removed from the water. That you also throw out some of the excess growth also can mean the nutrients bound up in the plant bodies are removed and will not be released back into the water. So their may be simply less of the nutrients available.
If the "tubes" you mention are some sort of plant food then another possibility could be it had a nutrient mix better suited for the watersprite than the fluval.
My thinking on the java fern is the loss of shade being the bigger factor. Where it grows well for me is in a shaded part of a tank
Good luck