Pipefish are very similar to seahorses in that they need to be in a species only or be the most aggressive fish in the tank. You can keep the mandarin and the pipefish, but anything else with it will bully and out-compete it for food.
Most saltwater fish have a minimum tank requirement. Pipefish isn't very highm and niether are mandarins, but a 30 gallon tank is very limited on the type of fish that can be kept safely and happily. More often that not your bioload is nowhere near the maximum, but that your tank is too small for the inhabitants. Live rock when fully cured does wonders for natural filtration.
Most saltwater fish have a minimum tank requirement. Pipefish isn't very highm and niether are mandarins, but a 30 gallon tank is very limited on the type of fish that can be kept safely and happily. More often that not your bioload is nowhere near the maximum, but that your tank is too small for the inhabitants. Live rock when fully cured does wonders for natural filtration.