First of all, I didn't have any experience on fish keeping before. But my child really wanted to have some fish, so I bought a small (0.5 gallon) fish tank.

I got him two guppies and two gold fish. But the gold fish died two days after I put them into the tank. I did some research online, and people said I have to change the water to keep it clean and maintain enough oxygen. I usually put some tap water in another tank, and keep it for 3 days, and every time I change half of the old water I clean the tank thoroughly. But every time after I change the water, it becomes dirty again soon after, and the fish swim on the top slowly, just like they're lacking oxygen.

I put some salt into the tank, seems help a bit, but not much. I don't know what to do. Do you think I need to use a pump to pump air into the tank? Will any pump fit that small of a tank? Do you think putting some sea weed will help? If so, what kind of sea weed should I put?

  • Is that US gallons? Your question and most of the answers don't clarify. Mar 25, 2014 at 18:30
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    Get at least a ten us gallon tank for the guppies alone. goldfish can reach anywhere from 20cm to 60cm, depending on the variety, so a 100g tank is minimum for the smaller, fancy varieties.
    – user1245
    Apr 26, 2014 at 10:24
  • I wouldnt have such a small tank as it is bad for the fish
    – user3769
    Mar 15, 2015 at 20:02

2 Answers 2


This is a good example of overstocking a fish tank. Don't feel too bad about it, I don't know anyone who didn't overstock their tank the first time. It's just too tempting.

Here's the problem though...

Fish waste contains ammonia, which is toxic to fish. Which wouldn't be a problem for them in a lake or river, but we keep them in a glass box. Meaning, there's no where for their waste to go.

That's why we do regular water changes. To replace some of the old, dirty water. It's also to keep the chemicals in the water balanced, or some bad chemicals can become too much for the fish to handle. I am glad to see that you're distilling your water before you use it, but I suggest going a little further and looking up (or starting a new question) how to cycle a fish tank, and why it's important.

Basically, in terms of what happened in your tank, there was a spike of ammonia from the fish waste that killed the goldfish. The ammonia came from the fish waste. Normally, in a cycled tank, the bacteria can decomposed the fish waste that isn't caught by the tank's filter. But in that small of a tank, there simply isn't going to be enough bacteria, and the tank's filter isn't going to be powerful enough to keep up.

Keeping fish requires a little bit of chemistry, and simply put, the less water you have in the tank, the less room you have for the chemicals that can harm your fish to dissipate.

Here's how I like to explain it: If you were to put a drop of food coloring in the tank you have now, it would quite noticeably alter the color of the water. Now put a drop of food coloring in a 10 gallon tank, and it won't change the color nearly as much. Put a drop of food coloring in a 55 gallon tank and you probably won't notice at all.

A good rule of thumb is for every inch of (adult) fish, you need to have 1 gallon of water. This not only balances out enough room for them to swim, but enough water to keep any dangerous chemicals from overpowering the water before you can fix it.

So, following that rule, a half-gallon tank will fit a half-inch fish. A full grown guppy will get about an inch long, and a goldfish will get about 12 inches long. That's 26 inches of fish.

You still have two guppies, which aren't going to live for very long unless you get them a larger tank. They are pretty hardy fish, so you might be able to keep them alive for a while, but it's going to take quite a bit of effort on your part. Remember, they don't have that much water to dissipate any chemicals that are harmful to them.

Here's my suggestion. Get a 10 gallon tank for the guppies, or donate them back to the fish store. They might live for a while in that small tank, but they are a schooling fish, and need at least 4 or 5 fish in the school to be happy. Not too mention you're going to be fighting too much to keep the tank clean (and the fish alive) for you to enjoy having them. And there's simply no point in keeping fish if you don't enjoy it.

So the tank is too small for fish, but that doesn't mean it's too small for anything. Most fish-keepers who get those small tanks get them to sat up a nano tank for shrimp. It's a big enough tank that you can keep a couple of Cherry Shrimp, Blueberry Shrimp, and many other interesting kinds of shrimp.

Another option, is an African Dwarf Frog. Though not very interesting in my opinion, kids seem to like them.

I knew someone who just kept a small tank of snails. Again, they don't interest me, but there are some pretty neat ones, like the Tricolor Horn Snail, or the Zebra Nerite Snail.

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    Thank you so much for the detail explanation and suggestion. I just bought a used 3-gallon tank which comes with a air pump. I put 4 fishes in it and have the tank filled up to 2.5 gallon. It seems work pretty good. The fishes look pretty active now. Let's see how long does it take for the water becomes dirty. BTW, one of my friends suggest me to install a filter. I wonder is it really needed? and what filter do I need for the tank like Hawkeye 2 Gallon 360 Starter Aquarium Kit. I also wonder do I need to grow some sea weed? They are petty expensive and I have no idea what to grow. Jan 14, 2014 at 7:56
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    @user1285419 I would't keep fish without a filter. For a 3 gallon tank, I would suggest getting something like the whisperlite: amazon.com/Tetra-25816-Whisper-BioScrubber-10-Gallon/dp/…
    – Spidercat
    Jan 14, 2014 at 9:05
  • @user1285419 As for plants, They're pretty easy to keep, I've never had any trouble with them, I just put them in and forget about them. Plants like Anacharis and Hornwort are pretty common, they grow pretty quickly so you'll probably find yourself throwing away overgrowth. Bamboo will survive underwater if you like how that looks. Otherwise a single, small, Anubias or arrowhead type plant would do fine I think.
    – Spidercat
    Jan 14, 2014 at 9:10
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    @user1285419 Just be careful what you mean by "dirty": the ammonia and other toxic fish wastes will be lethal to your fish long before the water is visibly "dirty". The filter is necessary to deal with that waste rather than keeping the water clear.
    – toxotes
    Jan 14, 2014 at 23:42
  • Just a couple small points - Surface area counts much more than gallons of water, which is why a 20L tank could be more heavily stocked than a 20H tank. Additionally, it's not advisable to keep goldfish in the same tank with tropicals.
    – JohnP
    Feb 14, 2018 at 16:53

The first, and possibly most important, thing to look at is your tank size. In my opinion, .5 gallons is way too small of a living space for 4 fish. If you have room for a larger tank, I recommend it. A bigger tank means more surface area, which means more contact with the air.

The most basic way to introduce oxygen to a still body of water (such as your tank) is to break the surface of the water. This can be as simple as adding a filter. The filter that I have drops the filtered water back onto the surface of the tank, which helps put some oxygen back in.

There are also a number of decorations you can put in to help oxygenate your tank. I'm sure you've seen them. Isn't everyone familiar with the treasure chest that opens occasionally and spits a few bubbles out? I see aerating decorations and filters all the time when visiting the fish aisle at my pet store.

To address the plant issue: this may or may not be desirable for your tank; it's up to you to decide. The problem is the balance of oxygen and carbon dioxide. While the plants will provide oxygen, will it be enough for the number of fish and tank size? Some plants also don't live very well in oxygen-rich tanks. This would limit the amount of extra aeration (filters, decorations, etc) that you have. It may be easier to start with one of the other methods of oxygenating.

You may also find this article interesting reading.

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