This is kind of urgent - I'm watching someone's dogs currently because that is what I am experienced in but they also have an aquarium. I have little to know experience with fish and I did warn the owners. I don't have 100% of the information, a lot of it is guesswork but I really need help.

In order to feed the fish, I dissolve krill in ice into a cup of water and distribute it. I feel so dumb, but for the first 4 or so meals, I used about 3 cups of Crystal Springs water from a water cooler to feed them. I'd say this tank is a little smaller than 125 gallons and I'm fairly sure it's saltwater because of the breeds of fish. Since then, I've fed them about 4 or 5 times just using the tank water (I feel bad for not realizing that at first, I know virtually nothing about caring for fish).

The owners come back in about 5 days, is there anything I can do to fix this? How harmful will this be to the fish? I've already noticed the gravel is dirtier than before. I also haven't been able to contact the owners because they have no cell connection where they have traveled to. Any advice would be greatly appreciated

  • 1
    If you can get a picture of the fish, we can tell you if it is salt or freshwater. That will slightly change my answer either way.
    – Henders
    Sep 28, 2018 at 16:07
  • No problem! The lights are on a timer so I've gotta wait a bit, but I know it includes a blue and a yellow tang as well as some clownfish and shrimp
    – fishnewb
    Sep 28, 2018 at 16:36
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    Yep, that's saltwater then :)
    – Henders
    Sep 28, 2018 at 16:38
  • Fresh water is what you add to a salt tank to replace evaporation ( salt does not evaporate). Sep 29, 2018 at 15:46

2 Answers 2


Don't panic, it's probably okay!

The key thing here is that you caught the error and asked the question. I'll cover both bases here and try and explain a few things (forgive me if you already know them).

Firstly, this is a good sized tank, so if you've only added a small amount of water, it probably won't make much difference. The dilution of the bottled water in the tank will be pretty high. Bottled water is also unlikely to be an issue to the fish because it normally contains less contaminants than regular tap water does - it's hard to tell unless you provided the chemical composition of the tap water.


Generally aquariums for Marine and Tropical fish need to be kept at a reasonable temperature for the fish to survive. Adding cold water (especially chilled) will lower the overall temperature of the tank. Again, this is mitigated by the fact the tank is 125 gallons so very little change will be felt by the fish.

As a side note - It is normally temperature extremes or sudden change which is most problematic for fish.

Saltwater Dilution:

If it is a salt water aquarium, it will have a specific salinity. This means how much salt is dissolved into the water. By adding freshwater to the tank, you've likely adjusted the salinity level of the aquarium. However, (have you spotted the theme yet?) the larger volume of the tank means that you have a much smaller effect on the water chemistry.

Dirty Gravel:

I wouldn't worry about the gravel. Most fish keepers will do a water change every week (somewhere between 10-25%) and the gravel will be cleaned. Likely, all you're seeing is the accumulated dirt since the tank was last cleaned.

Don't feel silly, why would you know?

Caring for fish is tricky, you shouldn't feel silly for adding water to the tank. The most common cause of fish dying while owners are away is overfeeding. In reality, fish only need a very small amount of food. When I go away for less than 5 days, I would just let them use their reserves (not that I'm advocating it in this case).

If I'm doing frozen food, I'll always use tank water to defrost the food. Just keep doing that and let the owner know when you can get hold of them.

  • 1
    Thank you so much!!! The frozen krill was thawed while in the fridge so it was cold but I think that's what the owners do normally, that's what they did when they showed me. It's been about four days so far since they left, I'm glad I have time for this to settle itself out. Thank you again!
    – fishnewb
    Sep 28, 2018 at 16:52
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    I just wanted add that the drop in salinity coming from the added freshwater is probably being mitigated by evaporation, unless this aquarist has a auto-top-off mechanism. And, just as with temperature, the dangers in salinity come from extremes or rapid changes, (which, as you said, are likely negligible due to the large water volume.) Sep 28, 2018 at 20:38

First, as Henders says, this is almost certainly not an issue at all. But I would continue to use tank water to thaw the krill from here on out.

In fact, I'd go a step further: you can put the tank water and krill in a small tupperware container and float that in the tank for several minutes. This not only thaws the krill, but gets it to the same temperature as the surrounding tank water.

I also wanted to add that the dirty gravel (if dirty means algae covered) could be an indication of overfeeding (which is very common for people who aren't in-the-know when it comes to aquariums). Uneaten food decomposes into nutrients that algae just love. It's possible that the fresh water you added contains phosphates, which are another thing that algae loves, and that this is the cause of the dirty gravel, but my money would be on overfeeding. For comparison sake, I feed my aquarium once or twice a week at most.

  • If you use the tank to heat the food by floating it inside, the tank still gets colder.
    – DonQuiKong
    Sep 29, 2018 at 6:34
  • @DonQuiKong: Typically, a saltwater tank would have heated water. In addition, again, due to the massive volume of water in the setup, the change in temperature is very small (but I admit I haven't done the math). Or maybe I misunderstood your point? Sep 30, 2018 at 19:10
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    well my point was, it doesn't matter how your warm the food, either by throwing it in or floating it inside, the water warming it loses the same amount of heat. Or do you want to heat it so the fish don't eat it while cold?
    – DonQuiKong
    Sep 30, 2018 at 19:15
  • @DonQuiKong: Yes, I get the food to tank temperature for the palatability of my fish ;-) I don't actually know if it matters to the fish/inverts at all. My thinking is simply that most things a fish eats in the ocean are cold blooded, so they are used to their food being "room temperature". I try to acomodate that. But I admit it may be overboard. Sep 30, 2018 at 20:17

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