I have several freshwater aquariums of following volumes:

  • one 5 gallon
  • one 10 gallon
  • two 20 gallon
  • one 29 gallon
  • one 55 gallon

and an outdoor 300 gallon pond. Aside from the pond, I started with one community tank and now have a rescue tank, a neon breeding tank and the others are holding tanks for my babies at different stages (the neon tetras, balloon mollies and fancy guppies just won't stop reproducing!) All of my tanks get weekly partial water changes, get tested with a Master Test Kit once a week, but vary in salinity, purpose and decoration - natural versus artificial plants.

All tanks have natural gravel except one. One of my 20 gallon tanks has black gravel. I have no live plants in this tank, just two Pygmy Cory cats and guppy babies. It's regularly cloudy, and yet the maintenance I perform on it is the exact same as the rest. I have zero fish loss, and my ammonia and nitrites never reach higher than 0.25 ppm and my nitrates never reach 5.0 ppm (usually test results between 0 and 5). All of my tanks have been running for 4+ years and I haven't had a fish die in 2 years, and that was because I accidentally smashed a baby guppy with my gravel vacuum during a water change.

I change my carbon every other month in all my other aquariums because I prefer to retain as much good bacteria as possible, but 3 months ago started changing this tanks once a month, as directed on the packaging hoping to increase clarity, but no change.

It's aggravating because it's an eyesore and I would know EXACTLY what to do if it were caused by shock, but it's not. Could this be because of the black gravel? It was rinsed before the tank was set up, but that was forever ago. I have concerns about doing a 100% water change and starting from scratch since my fish are in such great shape. Has anyone had this happen to them before, where all parameters were fine but the tank was cloudy? I'm stumped! Any help would be greatly appreciated.

Sorry for the book, I just wanted to put as much background info as possible.

  • what is the hardness of the water where you live,if the general hardness of the water is low you might have to use some gh salt to get the hardness of the water up to around 10-15degrees gh,and please provide a measurment of the ph of the water too. May 24, 2017 at 6:21
  • The water is pretty hard here, but I don't have a dh or KH testing kit, my master includes pH, high range ph-ive never had to use, ammonia, nitrites and nitrates. Should that be a factor? If so I'll order the kit. Just checked ph it's at 7.2 and is typically 7.2-7.4.
    – Christy B.
    May 24, 2017 at 6:38
  • It's not needed to filter over carbon continuously. This is only required after e.g. you treated your fish with medicine. Do you still have other filter media in your filter? Usually it contains some filter sponges and filter cotton. You almost never have to change the sponges. The cotton you can rinse a bit in aquarium water you took out. If you indeed change all filter material every month, then it could be that you have bacteria blossom (causes cloudy water). It's something you usually have with new tanks, but it can also occur when you replace the entire filter.
    – Diether
    May 24, 2017 at 10:58
  • I have cotton and foam, bio balls, ceramic cylinders, and the carbon pouch. I was hoping changing the carbon would help to clear the water, but my assumption was wrong. The only thing Ive changed is the carbon pouch. It's a marine land canister filter, if this helps in identifying any issues.
    – Christy B.
    May 24, 2017 at 12:50
  • having excluded soft water the reason for cloudy water is most likely biological in origin,as in a bacterial bloom the next thing to do is check the filter is the filter media covered in light brown bacteria that smells like wet soil,this means the filter is working correctly,if not taking filter media from one of your other tanks might help to seed the tank whith the helpful bacteria. May 24, 2017 at 14:17

2 Answers 2


What are the differences between the tanks? If the cloudy tank has fake plants in it and the others have real plants. You add a little salt to the real plant tanks. Water softerns in houses for well water use salt to make it less hard. Another idea would be during a water change instead of fresh water add water from one of the tanks that are clear. See if it stays clear or if it clouds back up. There are alot of variables that can contribute to this issue. I'd recommend taking changes slowly and only one change at a time. Put a week in between your trails, so you'll know what the problem was/is.

  • Artificial plants in both, same gravel in both, size is different.could it be placement? Dust or window light exposure?
    – Christy B.
    Jul 26, 2017 at 3:47
  • If you just let the tank ride, instead of changing water and whatnot, does it get more cloudy?
    – Dosvita
    Jul 26, 2017 at 3:58
  • I haven't really changed my habits with the water changes, so I don't know how to answer that, it has cleared up some since the last time I did a water change, added about 50%fresh treated tap and 50% from a different tank, possibly more established. Like u had said
    – Christy B.
    Jul 26, 2017 at 5:11
  • Also realized my filter was spitting tiny micro sized bubbles because the Amt of water in the filter had depleted some from the water change, adding water into the filter helped a little too bc the bubbles were creating a false, cloudy facade
    – Christy B.
    Jul 26, 2017 at 5:13

If you did buy the gravel/sand where you did buy your tank it is probably safe but if bought it at a garden center or at other sources where decoration sand/gravel is sold there might be problems as they might contain harmful substances(heavy metals or dissolvable toxic chemicals or even unwanted but non toxic minerals like calcium or salts). It is also best to use size 2-4mm gravel and avoid using the finer grained types of gravel/sand. The reason for this is that toxic gasses might build up in the sand. You might see this when cleaning the tank when small bubbles of gas are released and they contain sulfur dioxide or other toxic gasses.

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