Filter cups keep floating

[thesoulpatch]

So as the title says i have filter cups that float just enough for water to flow under them when ever i put new polyfil in. I need a way to hold them down. I don't have any extra rock to toss in them. Thoughts on how to keep them down?

 

[vetteguy53081]

So as the title says i have filter cups that float just enough for water to flow under them when ever i put new polyfil in. I need a way to hold them down. I don't have any extra rock to toss in them. Thoughts on how to keep them down?

Add a piece of coral rock or rubble to the cup
Alternative is a pouch of carbon