How to approach a change in substrate

So I've had my 180 set up for over 5 years now and haven't really had the opportunity to mess around with it too much. For the past few months, I've been wanting to change my current substrate, a pretty rough pea gravel over to black fluorite capped with black fluorite sand, but I've struggled to find any similar changes which haven't involved moving fish in and out of the aquarium. For a number of reasons, moving my livestock; a small shoal of rainbow fish and a single tiger plec, is not really feasible, and hence I'm going to need to do the change with the fish in. Initially, I wanted a smaller, finer substrate that would harbour the nutrients that my pea gravel so desperately lacked and enable runners to grow, seemingly unlike the current gravel, hence I thought fluorite would be ideal, given it also didn't give a huge ammonia spike when introduced. However, I've heard about the dustiness of the product, and am also concerned about the bacteria being removed as I remove my current gravel. Having stuck with the internal filter that came with the Juwel, I don't have many bacteria living in my filter, hence I was pretty concerned about new tank syndrome, so I was wondering whether gradual changes in substrate would help this, or whether holding the gravel in 'gravel socks' would also be useful. Ultimately, I'm looking to heavily plant and remain kinda low tech; no injected CO2 but I'll use root tabs and ferts where necessary, but the initial change over from gravel to fluorite is one that concerns me quite a bit. The last thing I want is a bunch of fish deaths I'd really appreciate some advice on how to approach this change, and whether it's feasible or not. Thanks.

 

I have sand in my aquarium and even though I rinsed it the sand was still dusty. I left my filter off for a few days so the sand could settle without clogging my filter. I think your best option would be to temporarily house your fish in another tank. Definitely move your old gravel over to avoid like you said, new tank syndrome. I would then transfer everything back once the sand settles with the "gravel socks". Do you mean put your old gravel in something to keep the bacteria but not the gravel?

 

I've done this in a 65g, this is the video I used-
https://m.youtube.com/watch?v=buMZbrbhJVs

You'll need several buckets to keep your old gravel in with aquarium water in. With your size tank I suggest rinsing your sand prior to doing the change otherwise it'll probably be an all day and night process. I'd do it all at once and keep gravel in some filter media bags while the sand is growing the bacteria.

 

Thanks for the advice. In terms of the gravel, I would be using gravel socks to try to maintain the bacteria that've grown over time, but I thought I should point out that the majority of the new substrate depth will be made up from general black flourite: the flourite sand will simply act as a thin cap so I've still got the option to house various corydoras/etc without damaging their barbs. I've heard a lot about the dustiness of flourite in general so I'll definitely be undertaking some serious cleaning prior to beginning the transfer. One thing I was wondering is what I can use a gravel sock.. As suggested, I was looking at filter media bags, but in terms of the scale, would I need to keep all 30+kg of gravel in a bunch of mesh bags or would it be safe to remove a set amount for convenience? Thanks again.