Advice on rescaping (adding sand) to 20 gal Ranchu tank

[Stef]

Hi folks. Looking for advice to remove small amount of existing gravel (4 lbs?) and replace with 20 lbs black Imagitarium sand for a Ranchu tank. Haven’t done this before, so don’t want to screw it up or stress out the fish. 
 

The tank is a 20 gal long. I was going to put the Ranchus in my 1st time 100 gal stock tank patio pond, but decided they were just too darn cute and delicate and I’d end up worrying myself to death over them. So decided to make their holding tank their official home but nicer. 
 

Being that the existing gravel is light colored, the fish will no doubt dig through new black sand and expose it. If I remove the light gravel I/they lose the beneficial bacteria. The existing light gravel is just enough to cover the bottom and you can even see the glass in spots, so maybe not even enough BB to worry about loosing. There’s two AC hob filters and two sponge filters now. The tank formerly housed baby BN Plecos that have since been rehomed. 
 

I plan to remove the fish to a 5 gal bucket while I add the sand and then reuse the same tank water. 
 

Thoughts?

[Janoš Bećar Pecaroš]

You can put the gravel in a mesh bag, like you have crushed coral in, and hang it next to the crushed coral for a month or so.