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Goldie was a birthday present in a bowl. That was our first mistake, and Goldie survived it, which is more than most bowl goldfish can say. Five years later she's in a 40-gallon tank with Snowy, Pumpkin and Peaches, still growing, and the tank is one of the calmest corners of the house. Here is everything I wish the pet store had told us, in the order you'll actually need it.
Quick answer
Goldfish need a big tank (20 gallons for the first fish, 10 more for each additional), heavy filtration, a cycled tank before fish go in, cool water, sinking food once or twice a day, and a weekly 30% water change. No bowl, no heater, no overfeeding. That's the whole hobby.
1. Tank size: bigger than you think
40 Gallon Breeder Aquarium
The size that finally stopped the algae fights. Wide beats tall.
Check price on AmazonA goldfish in a bowl doesn't stay small because it's happy. It stays small because it's stunted, and it usually dies young. Common and comet goldfish can reach a foot long. Fancies (fantails, orandas) stay smaller but are rounder and messier. The standard guidance is 20 gallons for one goldfish and 10 gallons for each after that. For four, that's 50; we're at 40 with heavy filtration and I'd go bigger if I had the floor space.
Shape matters too. Wide and long beats tall. A 40-gallon breeder is a great goldfish tank because it's 36 inches long and 18 wide. More surface area means more oxygen, and goldfish like oxygen.
2. Cycle the tank before the fish
This is the step that everybody skips and that kills the most goldfish. Fish produce ammonia. Ammonia is toxic. In an established tank, bacteria in the filter convert ammonia to nitrite (also toxic) and then to nitrate (mostly harmless, removed by water changes). Those bacteria take two to six weeks to grow.
How to cycle a new tank:
- Set up the tank, filter running, dechlorinated water.
- Add a pinch of fish food daily (it rots into ammonia), or a few drops of pure ammonia.
- Test every few days. Ammonia will rise, then nitrite, then nitrate.
- When ammonia and nitrite both read zero and nitrate is climbing, you're cycled.
- Do a big water change and add fish.
API Freshwater Master Test Kit
Liquid drops, not strips. Trust me, I read labels for a living.
Check price on AmazonGet the liquid test kit, not strips. Strips are vague. Liquid drops give you numbers. I'm a pharmacist; I like numbers.
3. Filtration: double it
Goldfish produce more waste than almost any other aquarium fish. Whatever filter the box says is right for your tank, get one rated for double. We run a hang-on-back and a sponge filter together. Full breakdown in the goldfish filter guide.
AquaClear Power Filter
Big media basket, easy rinse. The workhorse on Snowy's tank.
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Double Sponge Filter with Air Pump
Gentle flow, can't suck up a goldfish, basically unbreakable.
Check price on Amazon4. Water: cool, dechlorinated, changed weekly
Goldfish are cold-water fish. They're happiest between 65 and 74°F. No heater. In summer our tank creeps toward 78 and I float a frozen water bottle in it on the hot days; the thermometer tells me when.
Tap water contains chlorine or chloramine, which burns gills and kills filter bacteria. Every drop that goes in the tank gets conditioner first. One capful of Prime treats 50 gallons; a bottle lasts us most of a year.
Seachem Prime Water Conditioner
A capful treats a lot of water. One bottle lasts us a year.
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Digital Aquarium Thermometer
Goldfish like it cool. This tells me when the summer tank creeps up.
Check price on AmazonWeekly water change: siphon 30% out through the gravel (the vacuum pulls up the waste), refill with conditioned water at roughly the same temperature. Twenty minutes. Pumpkin follows the siphon around the whole time.
Aquarium Gravel Vacuum Siphon
Weekly water changes without the mess. Pumpkin follows it around.
Check price on Amazon5. Feeding: less than you think
Sinking Goldfish Pellets
Sinking pellets cut the gulping-air problem Peaches had.
Check price on AmazonGoldfish will eat until they hurt themselves. Feed once or twice a day, only what they finish in about a minute. Sinking pellets are better than flakes for goldfish because flakes make them gulp air at the surface, which can lead to swim bladder trouble. Peaches used to float sideways after meals until we switched. Problem solved in a week.
Once a week, shelled peas (frozen, thawed, skin off, squished) act as fiber and clear out the digestive tract. Blanched spinach or zucchini occasionally. Skip the freeze-dried bloodworms as a staple; treat only.
Fasting one day a week is fine and probably good for them. Nobody starves.
6. Tankmates and decor
Goldfish with goldfish. Tropical fish need warmer water and goldfish will eat anything that fits in their mouths. Smooth gravel or sand; goldfish scoop substrate and sharp gravel scratches their mouths. Avoid decor with holes a fish can wedge into. Live plants get eaten, so we use anubias and java fern tied to rock (tough enough to survive) and accept some nibbling.
7. Signs something's wrong
| What you see | Likely cause | First move |
|---|---|---|
| Gasping at surface | Low oxygen or ammonia | Test water, add airstone, water change |
| Clamped fins, hiding | Water quality or illness | Test water first, always |
| Floating sideways after eating | Swim bladder / air gulping | Sinking food, peas, fast a day |
| White spots like salt | Ich (parasite) | Raise temp slightly, treat, quarantine |
| Red streaks in fins | Ammonia burn | Big water change, test, check filter |
| Stringy white poop | Internal parasites or diet | Peas, then medicate if it continues |
Pharmacist note: almost every goldfish problem is a water problem first. Before buying any medication, test the water and do a water change. Medications in a dirty tank make things worse.
Our weekly routine
- Daily: feed once (morning), glance at the thermometer, count fish, watch for anyone acting odd.
- Every other day: second small feeding, or skip.
- Sunday: test water, 30% change with gravel vacuum, rinse HOB sponge in old tank water every other week, wipe glass, peas for everyone.
- Monthly: check impeller, trim plants, look at the filter intake for gunk.
Starter shopping list
- 40-gallon breeder (or the biggest you can fit)
- Hang-on-back filter rated for 2x the tank, plus a sponge filter
- Air pump and airstone
- Liquid test kit
- Water conditioner
- Gravel vacuum siphon and a dedicated bucket
- Digital thermometer
- Sinking goldfish pellets
- Smooth gravel or sand, a couple of tough plants
Everything except the tank is in the "Shop this guide" section below and in our Favorites.
Frequently asked questions
What size tank does a goldfish need?
20 gallons for the first goldfish and 10 gallons for each additional one. Bowls are too small; goldfish grow large and produce a lot of waste.
Do goldfish need a heater?
No. Goldfish are cold-water fish and do best between 65 and 74°F. A heater is only needed if your room drops far below that.
How often should I feed goldfish?
Once or twice a day, only what they eat in about a minute. Sinking pellets reduce air gulping. Fasting one day a week is fine.
How often should I change goldfish water?
About 30% weekly, using a gravel vacuum and conditioned water at the same temperature. More often if nitrate climbs above 20 ppm.
Why is my goldfish floating sideways?
Usually swim bladder trouble from gulping air with floating food or overeating. Switch to sinking pellets, feed shelled peas, and fast for a day. Test the water too.
Watch it in action: most of the gear in this guide shows up in our videos. Head to the channel and search the pet's name.
Shop this guide
40 Gallon Breeder Aquarium
The size that finally stopped the algae fights. Wide beats tall.
Check price on AmazonAPI Freshwater Master Test Kit
Liquid drops, not strips. Trust me, I read labels for a living.
Check price on AmazonSinking Goldfish Pellets
Sinking pellets cut the gulping-air problem Peaches had.
Check price on Amazon
