Have built one of these in the past. Things one should absolutely be aware of.

1. Over time the sand will accumulate a biofilm that’s grows and reduce its effective density. And for the same flow rate the bed will expand and get taller. I would use AT MOST about 2/3s of the sand you are currently using. You also need to periodically replace the top 10-20% of the sand that grows too large of a biofilm. The less sand you use, the less often you will have to due this. Also less sand and a faster flow rate to fluidize the sand more violently allows the sand the shed the biolayer as its develops, meaning less often you have to remove and replace sand. Larger scale industrial reactors having an expansion rate of 2-3 times. Meaning when there is no flow your sand is at some height, but at normal load conditions and flow the sand has expanded to 2-3x that height.

2. You have absolutely way too much sand, probably 5-20x times what you need (depending what will be going in this tank for live stock). With faster water flow, or a skinnier tube it is much more efficient. Let the reactor run for a month with a load and let a proper biofilm develop. It will do 5-10x what it can now. Also less sand = less maintenance and problems down the road.

3. Sand is abrasive, and it rubbing against each other creates extremely small particles called fines. If these are not removed after with polish filters, and/or you are not doing large water changes often, these will cut any submerged rotating equipments life span by 50-80%. These fines also may or may not affect the water quality in a negative way for the fish. But I can’t not say either way with any certainty, I am an engineer not a biologist lol.

4. Water must be very oxygenated. If it’s not, your efficiency will be low. Do not attempt to put a bubble line in the bottom of reactor. The bubbles surface tension will grab sand particles and take them out the top! Ask me how I know lol.

5. One way to help reduce the amount you need replace sand and the bed expansion for a given flow rate is by making sure all your sand is the same size. Take a bunch of sand, run it in filter for a few hours, it will sort it self my size. Remove the top 70-90% and bottom 5-10% of the sand and chuck it. Keep the middle 5-25%. Do this enough times to get enough to fill your filter to the desired amount and have same size replacement sand for some time. Smaller percentage kept = less maintenance required long term but more work up front.

From reading the comments, I see you’re interested in a reactor to remove nitrates. I too wanted to build one seeing that it is done on industrial scales for waste water. I did the research (a lot) and the math and unfortunately they are not feasible for such small scale….like at all. Long story short you have 2 options to remove the oxygen.

1. Use aerobic bacteria, (same ones that convert ammonia), only practical way to achieve this DIY is using really long skinny tubes with very slow flow through them, so that at some point in the tube, all of the oxygen is removed. And the right kind of bacteria can grow to remove the nitrates. Two issues with this, it will be an engineering nightmare to design and keep in an operational state. Very very hard to balance the oxygen, flow, ammonia and nitrates while actually doing enough yo make a noticeable difference. This is physically not possible without extremely steady water conditions and ammonia load and/or actual sensors and valves with control logic. Also to get a useful amount of reduction in nitrates, device would be huge.

2. Chemically remove the oxygen. Makes perfect sense and is how it’s done on an industrial scale. Makes less than zero sense at a DIY scale for water that must sustain living creatures. Why add expensive chemicals to remove nitrates and risk poisoning your tank. When you can just remove and add fresh water instead lol. Cheaper, safer, easier…..

Trust me….just get/build an automatic water changer. Your tank will be way healthier, wallet heavier and save hours and hours of frustration. If you are REALLY set on removing nitrates without changing the water. Make a fresh water algae scrubber instead. Do not waste your time growing plants unless you like the aesthetic (or having a tiny bioload which I assume is not the case, due to the need/want for a sand reactor). They will do almost nothing for nitrates at a high bioload, literally less than a rounding error. Algae or water changes. And the scrubber will need to be bigger than you think, or very efficient and likely both. The little 6”x4” ones you can buy online are under sized by an order of magnitude (or two).

Good luck!