Hi,
I've finally got the planted half of my tank sorted and it's got a black grit substrate. the other half is in white silica but because i've been planting and moving things around there's now a lot of black grit mixed in with it. I've got a ton of silica left so I thought I'd just syphon the current silica/grit mix out and replace with fresh silica.
I know this will have an effect on the bacteria in the tank but will be a killer? it's approximately 50% of the substrate that will be replaced all up.