Goldfish and Comets: What Every Beginner Needs to Know Before Fish Die

If you just brought home goldfish from the pet store — or grabbed a few from the fair — you're in good company. Comets and common goldfish are the world's most popular first fish. But they're widely misunderstood, and most beginner goldfish don't survive past the first month.

Quick Answer: Comets are a variety of goldfish (Carassius auratus) with a long, forked tail. Both goldfish and comets need at least a 20–30 gallon tank, a working filter, and a fully cycled aquarium. They're tough fish — but the wrong setup kills them fast. Cycle first, size up, and they can live 10–15 years.

What Are Goldfish and Comets, Exactly?

Comets are a type of goldfish — the same species, just a different variety. Both belong to Carassius auratus [1]. "Goldfish" is an umbrella term covering dozens of breeds. Comets are one of the most common, and they're what most people picture when they think "goldfish."

Here's what sets comets apart from other goldfish varieties:

• Long, deeply forked tail — the "comet" streamer shape gives them their name
• Slim, streamlined body — built for speed, not show
• Can grow to 12 inches in a large tank or pond
• Colors: orange, red, white, yellow, or two-tone Sarasa comets

Common Myth: "Goldfish only grow to the size of their tank." Reality: A cramped tank stunts growth by causing organ damage and chronic ammonia poisoning [2]. The fish doesn't stay small and healthy — it stays small and sick. This distinction matters enormously for beginners.

How Comets Differ from Fancy Goldfish

Fancy goldfish (Orandas, Ryukins, Ranchus) have round, egg-shaped bodies and double tails. They're slower and more delicate than comets. Comets have single tails and slim bodies — they swim fast and compete aggressively for food.

For beginners, comets are the smarter pick. They tolerate temperature swings better than fancy varieties. They recover from early mistakes more easily. If this is your first tank, start with comets — not Orandas.

For a complete breakdown of comet-specific care needs, see our Comet Goldfish care guide.

Why Your Fish Keep Dying (And What's Actually Happening)

If your goldfish are dying, new tank syndrome is almost certainly the cause. This is the #1 killer of beginner fish — and most pet stores don't explain it [2].

Here's what happens inside a new tank:

• Fish produce ammonia through waste and breathing
• Ammonia is toxic — just 0.5 ppm can kill fish
• Beneficial bacteria in your filter convert ammonia → nitrite → nitrate
• This bacterial colonization takes 4–6 weeks to complete

A brand-new tank has no bacteria yet. Add fish immediately, and they swim in rising ammonia. That's new tank syndrome — and it looks like your fish "just got sick for no reason."

How to Cycle Your Tank Before Adding Fish

You have two real options here:

1. Fishless cycle (best option): Add a pinch of fish food or pure ammonia to feed bacteria. Wait 4–6 weeks. Test daily with a liquid test kit like the API Master Test Kit. When ammonia and nitrite both read 0 ppm and nitrate rises, you're cycled.
2. Fish-in cycle (if fish are already home): Do daily 25–30% water changes to keep ammonia below 0.25 ppm. Add Seachem Prime water conditioner with each change — it temporarily detoxifies ammonia and buys critical time.

Pro Tip: Skip the strip tests — they're notoriously inaccurate. Get the API Freshwater Master Test Kit. It tests ammonia, nitrite, nitrate, and pH. It's the #1 tool the keeper community recommends for beginners, and it's worth every cent.

According to PetMD's goldfish care sheet, water quality monitoring is the single most important factor in goldfish survival. Diet and tank decor are secondary.

Signs Your Tank Isn't Cycled Yet

Watch for these red flags in the first weeks:

• Fish gasping at the water surface (ammonia stress signal)
• Fins clamping tight against the body
• Fish hiding or sitting motionless at the bottom
• Red streaks or bloody patches on fins

If you see these signs, do a 30% water change right now. Don't wait. Don't add more fish until the full cycle completes.

Tank Size: The Mistake That Kills Most Beginner Goldfish

Goldfish and comets need far more space than most beginners expect. A comet in a 5-gallon tank is like a Labrador in a bathroom. It technically fits, but it won't last long.

Here's the sizing breakdown that reflects how goldfish actually grow [1]:

Each additional goldfish needs 10 more gallons. Two comets require at least 50 gallons to thrive.

Why Small Tanks Are Dangerous

Goldfish produce huge amounts of waste. In a small tank, ammonia builds up faster than any filter can handle. Water temperature also swings wildly in small volumes.

Goldfish prefer water at 65–72°F (18–22°C). A 5-gallon tank heats near a sunny window and chills overnight. Immune systems crash. Disease follows fast.

Pro Tip: Already bought goldfish before realizing you need a bigger tank? Do daily 25–30% water changes while you save for an upgrade. A 40-gallon starter tank runs $60–100 on sale — it's the single best investment for keeping your fish alive long-term.

Our complete guide to goldfish in an aquarium covers setup, filtration, and tank sizing with step-by-step instructions built for first-timers.

New to goldfish and feeling overwhelmed? Our beginner goldfish tank setup guide walks you through every step — from cycling to your first fish day. No experience needed. →

What Do Goldfish and Comets Eat?

Goldfish and comets are omnivores — and overfeeding is one of the fastest ways to poison your tank. Their eagerness to eat everything fools beginners into giving too much, too often.

Follow these feeding rules to keep water quality stable:

• Feed once or twice per day
• Offer only what fish eat in 2 minutes — remove the rest immediately
• Use sinking pellets, not flakes (flakes dissolve fast and cause goldfish to swallow air, leading to swim bladder problems)
• Fast the fish one day per week — goldfish digestive systems benefit from regular rest days

Common Myth: "Goldfish always beg because they're hungry — so feed them more." Reality: Goldfish have memories lasting months [3]. Their constant begging is learned behavior, not genuine hunger. Extra food rots and spikes ammonia. Feed less, not more.

Best Foods for Comets and Goldfish

Variety keeps goldfish healthy and colors vibrant:

• Sinking pellets — Hikari Oranda Gold is a trusted keeper-community favorite
• Blanched vegetables — shelled peas, zucchini, spinach (blanch first to soften)
• Frozen foods — brine shrimp, daphnia as occasional treats
• Gel food — Repashy Soilent Green is nutritionally complete and reduces tank waste

According to PetMD's complete goldfish care guide, a varied diet including vegetables improves immune function and coloration. Avoid cheap goldfish flakes as a primary food source — they break down too quickly and cloud the water.

For a full breakdown of nutrition and feeding schedules, see our goldfish food guide.

Can Goldfish and Comets Live Together?

Yes — goldfish and comets can absolutely share a tank. They're the same species, prefer the same water temperature, and have compatible temperaments in a properly sized tank.

But there are rules beginners often miss:

• Don't mix comets with fancy goldfish. Comets consistently outswim and outcompete Orandas and Ranchus. Fancy varieties slowly starve in a mixed tank.
• Match fish by size. Any fish small enough to fit in a comet's mouth is a potential meal. Start with similarly sized tank mates.
• Never add tropical fish. Goldfish need 65–72°F. Tropicals need 76–82°F. Long-term cohabitation harms both species.

Best Tank Mates for Comets

The safest companions for comet goldfish:

• Other single-tailed goldfish (common goldfish, shubunkins)
• Sarasa comets of similar size
• Weather loaches (widely reported as compatible by the keeper community)

According to The Spruce Pets' comet goldfish care guide, comets do best with similarly sized coldwater fish. Never house them with tropical species or delicate fancy varieties.

Fish to Keep Away from Comets

Avoid these tank mates entirely:

• Small fish (neon tetras, guppies, danios) — comets eat them
• Fancy goldfish (Orandas, Ranchus, Ryukins) — comets outcompete them at feeding time
• Aggressive species (cichlids, tiger barbs) — they attack and shred goldfish fins

Common Comet Goldfish Problems (And How to Fix Them)

Most goldfish health problems trace directly back to water quality. Before buying any medication, test your water. Poor water quality causes roughly 80% of visible goldfish symptoms.

Gasping at the Surface

This signals low oxygen or high ammonia. Act immediately: do a 30% water change, increase surface agitation with an air stone, and test ammonia with your kit. Don't wait overnight.

White Spots (Ich)

Tiny white dots like grains of salt are Ich (Ichthyophthirius multifiliis). Very common in new, stressed tanks. Treatment: slowly raise tank temperature to 78°F over 24 hours and hold for 10 days. Add aquarium salt at 1 tablespoon per 5 gallons.

Swim Bladder Problems

Fish swimming sideways or upside down usually have swim bladder issues. This is almost always diet-related. Fix: fast the fish for 48–72 hours, then offer blanched, shelled peas. Switching to sinking pellets prevents recurrence.

Fin Rot

Ragged, discoloring fins signal bacterial infection from poor water quality. Fix the water first — every-other-day water changes for two weeks. If it progresses, treat with Seachem Kanaplex as directed on the package.

As of June 2026, the keeper community consensus on r/Goldfish consistently ranks "water changes before medication" as the first response rule for any goldfish illness.

Pro Tip: Keep a simple log beside your tank. Record your water test results each time you test. Patterns in the numbers reveal problems before fish show symptoms. Most experienced keepers catch issues 2–3 days before they become a visible crisis.

Ready to set up your goldfish tank the right way from day one? Our complete goldfish tank setup guide covers every piece of equipment, the full cycling process, and week-one troubleshooting — built for beginners. Start safely →