Well, sir, your Fluval 405 canister filter is rated for aquariums of up to 100 gallons, so it will eventually be able to provide all of the biological filtration necessary for your 50-gallon seahorse tank all on its own.
But right now, that’s not what it’s doing. At this point, the live rock and live sand in your aquarium are performing a considerable portion of the biological filtration for the aquarium, and you are going to lose that additional biological filtration ability when you remove the substrate. Depending on how much live rock and live sand is in the aquarium, it probably accounts for these 50% of the biological filtration in the tank, with your Fluval 405 providing the remaining 50% of the biological filtration.
That means you will be losing approximately half of the biofilter when you remove the live rock and live sand, and the aquarium is going to be much less stable for some time afterwards. The biological filtration media in the Fluval 405 will eventually build up a larger population of nitrifying bacteria in order to compensate for that loss, but that’s a process that will take some time — at least a week or two. In the meantime, it will only be able to break down about half of the ammonia that is being produced in your aquarium, which means you will have to guard against harmful ammonia spikes for the first couple of weeks after you remove the live rock and live sand.
Monitor the ammonia and nitrite levels very closely, feed the seahorses and other specimens very sparingly during this period, keep the tank under stocked, and be prepared to perform water changes as often as necessary in order to maintain optimum water quality and correct the spikes in the ammonia and nitrite levels that are bound to occur while the Fluval 405 is adjusting to the change. Otherwise, your seahorses will be stressed and the high levels of ammonia and nitrite will impair their breathing and lead to all sorts of troublesome complications.
Also, you will need to be aware that when you remove the live rock and siphon out the live sand, you will be releasing quite a bit of detritus and sediment into the aquarium water. It may take the better part of a day for the mechanical filtration in your canister filter to clear up the water again afterwards, and the mechanical filters will need to be cleaned thoroughly or replaced after they have strained out all of the sediment and particulate matter that was stirred up when you removed the live rock and live sand. It would be a good idea to relocate the seahorses to your hospital tank or quarantine tank temporarily during this time, so that they are not exposed to the dirty water conditions and any bacteria that may have been stirred up and released from the substrate.
And of course you will want to retrieve the snails and micro hermits from your cleanup crew before you remove the live sand and live rock. The aquarium janitors and sanitation engineers won’t be very happy with a bare-bottom tank, since they like to burrow within or sift through the live sand and spend much of there time grazing on the macroalgae and cleaning the detritus from the live rock. But there will also be less need for the cleanup crew with a bare glass bottom that is more sanitary and makes cleanup and maintenance easier.
If you want to concentrate the hitching posts and decorations primarily in one corner of the aquarium, that’s fine, but you should realize that the seahorses are going to congregate where the holdfasts and shelter is and may pretty much ignore the rest of the tank. So I would be be inclined to distribute the artificial corals and plants throughout the aquarium, which will also assure that the seahorses move throughout the tank. Otherwise, with all of the plants and decorations confined to one area of the tank, which is where the seahorses will spend all of their time hanging out, the rest of the 50-gallon aquarium is going to look awfully barren.
Feel free to experiment with a bare glass bottom aquarium for your seahorses as long as you provide sufficient hitching posts and shelter for the seahorses, and allow plenty of time for the Fluval 405 to adjust to the heavier burden it will be carrying. Beware of the increased risk of ammonia and nitrite spikes after you remove the live rock and live sand!
Best wishes with all your fishes, Sean!