Nitrate spike!

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Jul 16, 2019
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Not sure if in qualifies as an emergency as all the fish appear normal and are eating well. I have a 75 gallon freshwater community tank with 12 fish total (2 Angelfish, 4 Cory's, 2 Rams, 4 Mollies), 2 live plants on driftwood (rest are decorations and silk plants)

Today before my water change I tested the water as usual and my ph has dropped more than usual and my nitrates spiked to a dangerous level.

- Nitrites 0

- Ammonia 0

- NITRATES 160 ppm (last weekly water change they were 20 ppm)

- Ph 6.6 and it is usually around 7.2 (I know it can fluctuate but I havent had it drop below 7 before)

- Tank Temperature runs at 81F

I have noticed that recently there has been some algae growth in the tank, nothing major and out of control but growing larger. My light is usually on for about 10 hours a day, then the blue light at night.

I am about to do my water change but wondering if anyone has some suggestions on how to get the nitrates back under control aside from the water change. I usually do 25% change out each week but will be doing a 50% due to the spike.

I have attached photos of the algae that has grown over the past week.
here are the photos


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In a word, organics are too high. This is due to insufficient water changes, too long a light period, and possibly other things not mentioned that include substrate vacuuming (in all open areas), filter cleaning frequently, and not overfeeding. The fish stock is not beyond the tank's ability so that is not a factor as it can be if the tank has more fish. The pH lowering is part of this organic issue, and frequently appears when nitrates rise.

Now to the solution. What is the GH, KH and pH of your source (tap) water alone? What is the GH of the tank? Provided these are not too far apart, a major water change is in order. Do 70-75% of the tank. Do the same tomorrow. Vacuum the substrate well during each. Make sure the filter is clean. The dark brown/black sludge you will see in the filter is the organics and that you want to get out of the system.

Do you have nitrate in your source water? Test it if you haven't already.

The pH difference will not hurt anything here, as it is minimal from 6.6 up to 7.2.

Reduce the daylight period (tank lighting) down to 8 hours, and use a timer so it is consistent. This has a profound effect on fish, and it will help with the algae/organics. And forget the blue light, which I hope is not on all night. This encourages algae (algae can use any light, unlike higher plants) and it disturbs fish. A period of total and complete black darkness of several hours is necessary every night. I just want to ensure this blue light is not on during that dark period which the fish and plants need to recharge themselves.

The photo of the it the greyish haze in the upper right area of the photo?
What I am confused about is I haven’t changed up my routine in the last year and I haven’t had an issue, the only thing that changed was last month I added the live plants and drift wood. I do my weekly 25 % water changes which I use a gravel vacuum, I move the decorations around to make sure I’m not missing any uneaten food. I have two filters on the tank running and both have 2 filter sponges in them and i alternate cleaning out the filter sponge in a bucket of the used tank water so each sponge gets cleaned out once a month (4 sponges, one each week). As for feeding I monitor the feeding and remove any access blood worms or shrimp that they have not eaten right away. I tried to keep my stock on the lower end to avoid poor water conditions and so far I haven’t had any issues.

As for the light Its on 10 hours a day (due to my shift lengths) but I will get a timer to have it going only 8, the blue light is on when we are using the tv room and turned off for the night. The tank was moved a couple months ago to a different room that does have a larger window but it is not in direct sunlight and we do usually keep the blinds closed during the day because of the tank.

The photo of the algae didn’t upload apparently it was too large. Grey haze on the back is just from the background and reflection I believe? The algae was just a few brown marks on the glass.

I do not have nitrate in my water (just did a quick test to double check) and ph is 7.8… I used my last tests for gh and kh on the tank and they were

KH 80
GH 120

I did a 50% water change before I saw this and I cleaned out all the filters. I will do another one tomorrow. Fish still look like they are doing alright, don’t look stressed but I know how fast that can change and I am trying to avoid that.

Thanks for all your advice!
The wood might have been a factor. Is the wood hard, or soft and sort of mushy/falling apart? I assume it is purchased from a fish store and not collected yourself?

Nitrate from 20 to 160 ppm is a huge increase though. BTW, Even 20 ppm is high, and given there is no nitrate in the source water and the minimal fish load for the tank, I would expect nitrate to be no higher than 10 ppm and probably in the 0 to 5 ppm range. Regular (once weekly) water changes of 60-70% of the tank volume should help.

Algae is natural and to be expected. Use a sponge or sponge scraper over the inside of the glass at each water change. There will be no signs of algae, but the biofilm will develop algae and doing this keeps it from appearing. I tend to only do the front glass.

Make sure you don't overfeed. The light reduction should help. And thanks for explaining.

BTW, I just spotted the mollies...this water is much too soft for this fish, it is going to have serious problems. Livebearers need mineral in the water, and mollies the most. They are also highly sensitive to any nitrogenous issue, ammonia or nitrate.
That amount of increase suggests that something is probably decaying in the tank. Your new driftwood or plants would seem to be the prime suspect.

BTW, I just spotted the mollies...this water is much too soft for this fish, it is going to have serious problems. Livebearers need mineral in the water, and mollies the most. They are also highly sensitive to any nitrogenous issue, ammonia or nitrate.
I agree, however it is worth pointing out that trying to change this will cause serious issues for all your other fish. Ideally these should be in their own tank and you can add minerals to the water to suit them. You could keep other live-bearers in that tank but none of your other current fish.
Did the water changes and the nitrates are back down. The wood doesn't appear to be mushy or falling apart. Are their any other signs that could make it the drift wood?

I replaced one of my filters to new to see if that was part of the issues. Didn't think replacing one of the two filters would cause much of an issues considering my tank is under stocked for the size? Could be wrong I have the old filters still incase I saw issues.

As for the mollies, the aquarium actually suggested them for my community tank (even took water samples, which I usually do before adding any fish to make sure they are compatible). They advised they would be good as they are suitable in a variety of water conditions. I added them a couple months ago and haven't had any issues with them as of yet. When about would I see issues arise? Maybe got some bad advice from whoever was working that day.

I don't want to change my water over because I got this tank a year ago specifically for the angelfish, the others I added just for some variety and colour. If the mollies are going to develop issues I will start cycling a new tank for them. I have my old 20 gallon that I had the angels in when they were babies.
One lesson everyone has had to learn - don't believe anything a shop says till you have researched it yourself. Too many workers either don't know or don't care and will say any rubbish to get a sale.
That's one thing I have slowly learned! From when I got my first fish and tank on the same day. However, the aquarium I go to now has been pretty good and seem well educated... that is until this mollie issue has arised.

Dragging my 20 gallon back out now
The store advice was as wrong as any advice could be, there is no point in pussyfooting around this. You asked when you would see issues...that is hard to say. What is now occurring is that the lack of mineral in the water (represented by the GH which is basically calcium and magnesium) is making their lives extremely difficult. This is weakening the fish, making them susceptible to other issues (like disease) that otherwise they could fight off. It causes stress, and stress weakens the immune system. It is hard to say how long it will be before you see external signs of this, but these include shimmying, wobbling in swimming, gasping at the surface, and lying on the substrate or other surfaces. Once it gets to the stage you see this, it is game over for the fish. So they are slowly dying, to put it bluntly.

I agree not to harden the water in the main tank as this then has effects for the softer water species. There is no middle road because the mollies need it quite hard.

The 20g is better (especially if it is a long rather than a normal high 20g) if you can raise the GH in the water. My suggestion would be to do this by using a sand that is calcareous, such as an aragonite/crushed coral mix. CarribSea make one. You do NOT want a marine sand as this also includes salt, and while salt is easier for mollies than other fish it is not necessary and best avoided. The aragonite/crushed coral sand is usually aimed at rift lake cichlid tanks, but it works very well for livebearers. I did this many years ago for mollies.

Using mineral salt preparations (again, those for rift lake fish, not "salt" meaning common or sea salt) is another method but then you need to buy the preparation and prepare water for water changes outside the tank. This can get involved and expensive. The substrate lasts for ever.

To the question of the wood, one way to test this is to remove the wood from the tank for a period of time and see if nitrates still rise. The wood can bee kept submerged to keep it waterlogged, in an empty tank or bucket or plastic container (any of these used ONLY for fish tank things). I would see if nitrates rise now, with a few major water changes, and if they do then remove the wood and see what happens. Wood normally should not do this, but we don't know where this wood comes from or what it may contain, so it is one possible.

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