First off, I'd be doing MAJOR sized water changes now. Like another 100L.
My first thoughts were that you have gradually attained water conditions that are causing the problems. Dirty water that supports expansion of nasty bacteria can occur over a period of time, not just in the near future.
For me, I quit using canister filters more than a decade ago because they ARE efficient at trapping the detritus, BUT, they are also not so convenient for cleaning out so I tended to let things go too long between cleanings. I am a firm believer, in seahorse tanks, that all mechanical filtration trapping food/detritus be cleaned out VERY frequently, and for me that's for most of my tanks, about 5 days, but in NO case let it go beyond 7. The trapped crap provides the food and bedding for those nasty bacteria that do so much damage to our seahorses. I find the HOB's to be Much easier to clean out and therefore less likely to be let go too long. IMO, you want to remove the trapped material BEFORE it decays to any great degree.
I also believe that for the long term health of the tanks that larger and more frequent water changes (like 45-50L weekly) and tank cleanings (removing all TRAPPED and hard to see detritus) are a great way to help keep the water quality up. As there are NO test kits available to us to tell us when the water has degraded to the point the bacteria have gotten out of hand, preventive maintenance is the best tool I have to help in that matter.
I guess there is an outside chance that the new sponge contained some contaminant but as it's sold for the aquarium trade I would think a good rinse first should be all it would need.
More than a decade ago, I used Rowaphos in a seahorse tank for the first time and lost all four seahorses in that tank and never used it again. Some years later I came across a post on seahorse.org that talked about Rowaphos use but I don't remember now what all it said. I've never had a problem using it in my reef tanks over many years.
