Hey, congratulations on the new dwarf seahorse babies!
It’s true that large amphipods can predate newborn seahorses, and you did well to remove the larger Gammarids that abandoned the sponge filter when you turned off the air supply. That was a wise precaution for a dwarf seahorse tank.
But I would return the sponge filter to your dwarf seahorse tank even if there are still a number of amphipods embedded in it at this point. Once the sponge filter is operating again, the amphipods will probably be quite content to remain within the sponge where they are provided with a continuous supply of freshly oxygenated water and a never ending buffet of choice detritus and dead brine shrimp nauplii that accumulate in the foam material via the mechanical filtration the sponge filter performs. The sponge filter thus provides them with ideal conditions and free room and board — plenty of shelter plus all the food they can eat, so hopefully there will be no need at all for them to stray from their spongy patch of Paradise and molest the baby seahorses.
Rinsing the sponge filter in saltwater shouldn’t disrupt the nitrifying bacteria it houses, so your biological filtration shouldn’t be impaired and you shouldn’t have to worry that the tank will crash.
If you want to be absolutely certain that the amphipods won’t present any risk to the newborn dwarfs, you can always remove the babies to a separate nursery tank for rearing. In the case of dwarf seahorses, this could be just a bare-bottomed 2 gallon glass aquarium filled with water from your dwarf tank and equipped with a coarse airstone and a number of suitable hitching posts for the babies.
Cured ”seahorse trees” make good hitching posts, as do artificial aquarium decorations such as small seafans and soft plastic plants with fine, branching leaves (Giwojna, Jan. 1997). Strips, sections, and cylinders of plastic window screen or the plastic mesh sold in craft stores for needlepoint projects also work well for benthic seahorse fry. Short lengths of polypropylene rope (the kind sold at hardware stores and marine outlets for boating purposes) are another good option for hitching posts in the nursery. They come in many different colors, can be cut to any desired length, and are buoyant so if one end is anchored and the other end is unraveled, they will wave gently in the current like natural plants. (Avoid nylon rope, however — it bleeds in saltwater and will leech color and who knows what else into your tank!) If necessary, the holdfasts can be secured to the bare glass with silicone aquarium cement or suction cups designed for use in marine aquaria, or secured to a piece of coral rubble to anchor them in place.
In this type of simple nursery setup, you stay on top of water quality by making small partial water changes a few times every day while using a length of airline tubing to siphon fecal pellets and dead or uneaten rite shrimp from the bottom of the tank. Nurseries for benthic fry such as dwarf seahorses don’t need to be fancy at all. (Heck, back in the day, my nursery tank for my first dwarf seahorses was an empty mayonnaise jar and even that did the trick as long as I performed partial water changes very regularly in conjunction with siphoning the bottom clean.)
But if you wanted to set up a somewhat more sophisticated nursery tank in anticipation of regular broods of dwarf fry, then you might consider setting up a divided nursery instead. The basic Divided Nursery tank designsimply involves separating a standard 10-gallon aquarium into two or more different compartments with a common water supply using perforated tank dividers. All of the equipment and filtration goes into one of the resulting compartments while the other compartment(s) serve as the nursery or nurseries for the fry. The perforated barrier allows water to circulate freely between the compartments while acting as a baffle that greatly dampens the turbulence generated on the equipment side.
It is also very effective at keeping newly hatched brine shrimp confined to the fry’s nursery compartment, especially if two or three of the perforated plastic dividers are sandwiched together side-by-side with a small 1/8-1/4-inch gap between them, forming a double barrier (Abbott, 2003). For best results, I would go one step further and cover the perforated tank dividers with plastic window screen or better yet the plastic mesh sold in craft stores for needlepoint projects to increase the effectiveness of the barriers (Abbott, 2003). Then I would darken the equipment side and position a strip reflector or table lamp at the end of the nursery compartment opposite the filtration side, in order to draw the baby brine shrimp (bbs) away from the tank divider and filters, while concentrating the bbs in a smaller area so the fry can feed more efficiently (Abbott, 2003).
All of the gear is thus isolated on one side of the partition safely away from the fry and their food. The larger volume of water a divided tank provides gives the nursery greater stability as far as fluctuations in temperature and pH go, makes it easier to maintain optimum water quality, and increases your margin for er for best results ror accordingly (Abbott, 2003). With the tank divided in this way, any sort of mechanical, chemical or biological filtration you care to provide can be safely operated in the equipment area without disturbing the delicate fry in the nursery area (Abbott, 2003). The developing young thus enjoy all the benefits that better filtration and a large water volume can provide, while being confined in a smaller nursery compartment, making it easy to maintain an adequate feeding density (Abbott, 2003).
To provide efficient biofiltration for the divided nursery, I would install a fluidized sand filter on the equipment side. Fluidized bed filters use a fine sand media suspended in upflowing water currents that provides a tremendous amount of surface area for beneficial bacteria to grow, and allows the filter to nitrify large amounts of ammonia while maintaining high overall water quality and stability. Once the fluidized bed has cycled, there will be no ammonia or nitrite spikes with such a nursery.
I know a number of hobbyists who use divided nurseries or variations on this theme for raising benthic seahorse babies like hippocampus zosterae and H. capensis fry.
Best of luck with your prolific pipe-size pigmy ponies and their progeny, sir!