It's not recommended to put seahorses in small aquariums other than dwarfs in tanks around 5g.
Standard seahorses should be in minimum 29/30g for a pair, with an additional 15g for each additional pair.
The wattage of the Ice Probes is such that IMO, I doubt they will do much for a tank in the 30g size, (depending on ambient temperatures it might lower temps 2 to 3 degrees), and probably can't handle the range for temperate species that require 68° or lower temperature unless ambient temperatures are already close to the target range.