My son had some mollies in his tank that i ended up putting in the tank with polys. They were pretty big...next morning all were gone andy del looked like he would explode.
Sevs get thick using the length of the fish could be misleading just test your water frequently and adjust your water changes. Poor water quality could lead to hith. I wouldnt do a dither. Tank may look empty now but that sev and polys will fill it soon enough