[rayjay]

IMO it is STILL hard to give a proper answer as each hobbyist can have differing husbandry and feeding protocols that will affect how soon the water becomes a potential breeding ground for nasty bacteria.
There ARE NO TEST KITS available to the hobbyist to be able to tell when those conditions are becoming better to aid in the bacteria expansion so we have to just be extreme in our husbandry and water change frequency/volume.
In my early years of seahorse keeping I ended up determining that many problems I developed in the tanks, were not recent occurrences, but rather things that were an accumulation over a fairly long period of time. Skip a cleaning or water change here and there and the water gradually degrades until all of a sudden you can actually see the problem via the seahorses themselves. As I'm basically a lazy person, this was hard to overcome.
The skimmer is an excellent aid, better when larger though.
Sand beds can be gravel vacuumed maybe 25% at a time weekly to help remove potential feeding/bedding grounds, and having mechanical filtration, especially like a sock on the overflow, cleaned at least once a week but more often is better before the trapped particulate matter starts to decay. I found that with the rock, it trapped an enormous amount of uneaten food and had to take a powerhead to blow off the rockwork and allow for settlement before vacuuming out, to at least lessen the amount of decay potential.
If you DON'T have a sock on the overflow, I'd advise finding a way to remove any uneaten food/detritus that settles in the sump compartments as that too will add to the potential bacteria problem.
Lastly, for a 50g tank you may be overloading it somewhat but that can be partially mitigated with a little more frequent and larger water changes than if it was just seahorses. (I personally have seahorse specific tanks as when I started I ended up with problems with adding pathogens the seahorses didn't do well with from adding other fish)