Advanced saltwater fish

The marine fish rated difficult in our catalog fail in predictable ways. Wild mandarins and the banded and bluestripe pipefish depend on live copepods, while captive-bred mandarins and most other pipefish take frozen food. Seahorses lose out to faster tank mates, the big angelfish outgrow the tank and pick at coral, and the cleaner wrasses rarely settle. They belong in a mature tank with a keeper who already has a reef or two behind them. Each profile says what the species demands and why.

Tank size: 5–396 gal; 5 gal (20 L) to 396 gal (1,500 L); half the list needs 100 gal (380 L) or more
Temperature: 73–81°F; 23–27°C, most of the list
pH: 8.1–8.4; Most of the list
Temperament: 26 peaceful; 42 mostly peaceful · 13 aggressive
Reef safety: 17 reef safe; 37 reef safe with caution · 26 not reef safe

81 species

Feeders

The fish that starve

The green and spotted mandarin dragonets are marked difficult for one reason: they eat copepods all day and rarely learn to take frozen food. Their profiles list 53 gallons (200 liters). The real requirement is a mature tank with live rock and a refugium growing pods faster than the fish eats them. The bluestreak and Hawaiian cleaner wrasses eat parasites off other fish and often refuse anything else. The leopard wrasses need constant small invertebrate prey and a sand bed to sleep in. The harlequin filefish eats Acropora polyps and little else, and the copperband butterflyfish, 7.9 inches (20 cm) in 79 gallons (300 liters), starves in a tank that cannot offer it worms and pods.

Seahorses

Seahorses and pipefish want a tank of their own

Seahorses lose every race for food, so they live with nothing faster than themselves. The lined seahorse reaches 7.1 inches (18 cm) and lists 29 gallons (110 liters) at 68–79°F (20–26°C), cooler than a reef, and White's seahorse runs cooler still at 68–75°F (20–24°C). The Brazilian and yellow seahorses are the tropical ones. Captive-bred lined, Brazilian, yellow and White's seahorses eat frozen mysis at least twice a day as adults, and young ones more often. The dwarf seahorse eats live baby brine shrimp several times a day. All of them want low flow and something to hold, and do best in a tall tank. The banded pipefish is 7.5 inches (19 cm) in 53 gallons (200 liters) and the bluestripe pipefish 2.8 inches (7 cm) in 26 gallons (100 liters); both hunt pods and settle only in a mature tank.

Big angels

The large angelfish outgrow the tank

The emperor angelfish reaches 15 inches (38 cm) and lists 185 gallons (700 liters). The French angelfish reaches 16.1 inches (41 cm) in 201 gallons (760 liters), the queen angelfish 17.7 inches (45 cm) in 251 gallons (950 liters) and is marked aggressive, and the blueface angelfish 15 inches (38 cm) in 219 gallons (830 liters). The koran angelfish reaches 15.7 inches (40 cm) and lists 185 gallons (700 liters). The majestic angelfish is a size down at 11.8 inches (30 cm) for 145 gallons (550 liters). All of them are marked not reef safe. The regal angelfish is smaller at 9.8 inches (25 cm) for 124 gallons (470 liters), with a coral caution rather than a no. It is here because it is a famously poor feeder. The rock beauty, conspicuous, flagfin and bandit angelfish on this page carry a coral caution too.

Tangs and giants

Tangs, unicornfish and the fish that need a wall

The Achilles tang is marked difficult and aggressive at 9.4 inches (24 cm) for 180 gallons (680 liters), and it is the tang most likely to come down with ich in a new tank. The powder blue tang is the same story at 9.1 inches (23 cm) in 124 gallons (470 liters). The clown tang reaches 15 inches (38 cm) and lists 238 gallons (900 liters). The blue hippo tang is mostly peaceful but reaches 11.8 inches (30 cm) and lists 180 gallons (680 liters). The bluespine unicornfish reaches 23.6 inches (60 cm) and lists 396 gallons (1,500 liters). The Moorish idol is peaceful and beautiful and marked difficult because most of them refuse to eat.

Predators

Morays, groupers, boxfish and the big puffers

The ribbon eel reaches 39.4 inches (100 cm) and lists 66 gallons (250 liters), and it is marked difficult because most refuse food and waste away. The honeycomb moray reaches 70.9 inches (180 cm) and lists 291 gallons (1,100 liters). The lyretail grouper reaches 31.5 inches (80 cm) and eats anything that fits. The map puffer reaches 23.6 inches (60 cm) in 238 gallons (900 liters). The longhorn cowfish reaches 18.1 inches (46 cm), lists 250 gallons (946 liters) and can release a toxin when stressed that kills the whole tank. The clown triggerfish is marked aggressive at 19.7 inches (50 cm) for 300 gallons (1,135 liters).

By the numbers

Smallest

Largest

Most peaceful

Fits a 30 gallon tank

FAQ

Common questions

What makes a saltwater fish difficult?

Feeding, size or temper. Wild mandarins, the bluestreak cleaner wrasse and some pipefish depend on live food. Captive-bred mandarins, Hawaiian cleaner wrasses and several pipefish take frozen food. The seahorses lose to faster tank mates. The Pomacanthus angels, the queen angelfish and the tangs list 124 gallons (470 liters) and up. The flagfin and bandit angelfish list 90 to 100 gallons (340 to 380 liters), and the rock beauty angelfish 140 gallons (530 liters). The morays and groupers eat their tank mates.

Can a mandarin dragonet live in a small tank?

Its profile lists 53 gallons (200 liters), but the tank also has to be mature. It must grow copepods faster than the fish eats them, which usually means a refugium. A young tank starves a mandarin at any size.

How big a tank does an emperor angelfish need?

185 gallons (700 liters) for a fish that reaches 15 inches (38 cm), and it is marked not reef safe.

Are seahorses hard to keep?

They are marked difficult. They need a tank of their own with low flow. Captive-bred adults eat frozen mysis at least twice a day, and young fish more often. The dwarf seahorse eats live baby brine shrimp several times a day. The lined seahorse lists 68–79°F (20–26°C) and White's seahorse 68–75°F (20–24°C), cooler than a reef.

Which tangs are difficult?

The Achilles, powder blue and clown tangs, which are marked aggressive as well, plus the blue hippo tang and the unicornfish for their size. The yellow tang is not on this list.

Do any of the difficult fish stay small?

The mandarins stay at 2.8 to 3.1 inches (7 to 8 cm) and the ruby red dragonet at 2.4 inches (6 cm). The bluestripe and narrowstripe pipefish are 2.8 and 3.1 inches (7 and 8 cm), and the dwarf seahorse is 1.4 inches (3.5 cm). They are hard for what they eat, not for their size.

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