I'm at my wit's end here. I set up this tank last winter and nothing seems to be going right. Got all the right gear - fluval stratum substrate, two fluval 3.0 lights, an fx6 filter, and pressurized co2. Switched from thrive all in 1 fertilizer to GLA dry ferts a couple weeks ago, but no luck. Stocking's pretty decent too - 60 cardinal tetras, 25 phantom tetras, 1 SAE, and 3 dwarf chain loaches.
New growth on my plants looks great, but the old stuff just dies off. Leaves get this dark green/brown gunk all over them and then just start dying. Nitrates are through the roof - 60-80ppm - and I'm back to square one just a few days after a big 40-50% water change. I use a mix with mostly RO water since my tap water's super hard. Thought switching ferts would help, but nope. I've got some pretty easy plants in there - S. Repens, limnophila aromatica, ludwigia repens, jungle val, Zorchzon sword, AR mini, and blyxa japonica - but nothing's doing well.
I'm stumped. My plants do great in my 40g, but as soon as I put them in the 125g, they start showing these deficiencies. Despite plenty of ferts and those high nitrate levels, I just can't seem to figure out what's going on. Any advice would be a huge help.
Help with a struggling 125-gallon planted tank
Re: Help with a struggling 125-gallon planted tank
I've been dealing with this tank for months now, and I just can't seem to get it right. I've got a solid setup - fluval stratum, 2 fluval 3.0 lights, an fx6 filter, and pressurized co2. I switched from thrive all in 1 to GLA dry ferts a couple weeks ago, but nothing's changed. My stock is pretty simple - 60 cardinal tetras, 25 phantom tetras, 1 SAE, and 3 dwarf chain loaches.
My plants put out bright new growth, but the old stuff just dies off. It starts with this dark green/brown gunk on the old leaves, and then they just start dying. Nitrates are way too high - 60-80ppm - and they're back up again just a few days after a big water change. I use a mix with mostly RO water since my tap water is super hard. I thought switching fertilizers would help, but nothing's improved. I've got easy plants - S. Repens, limnophila aromatica, ludwigia repens, jungle val, Zorchzon sword, AR mini, and blyxa japonica - but they're all struggling.
It's weird because my 40g does great, but my 125g is just a mess. I've got plenty of ferts and high nitrate levels, so I'm stumped.
My plants put out bright new growth, but the old stuff just dies off. It starts with this dark green/brown gunk on the old leaves, and then they just start dying. Nitrates are way too high - 60-80ppm - and they're back up again just a few days after a big water change. I use a mix with mostly RO water since my tap water is super hard. I thought switching fertilizers would help, but nothing's improved. I've got easy plants - S. Repens, limnophila aromatica, ludwigia repens, jungle val, Zorchzon sword, AR mini, and blyxa japonica - but they're all struggling.
It's weird because my 40g does great, but my 125g is just a mess. I've got plenty of ferts and high nitrate levels, so I'm stumped.
Re: Help with a struggling 125-gallon planted tank
I think I see a lot of algae in your tank. The lighting might be too intense, especially with those two Fluval 3.0 lights. Maybe someone with more experience can give you a better answer.
Re: Help with a struggling 125-gallon planted tank
It's likely you're dealing with Blue Green Algae, also known as Cyanobacter bacteria - it's a photosynthetic bacteria that thrives on nutrients, low oxygen levels and red light. The high nitrates, fertiliser and CO2 in your tank are essentially fuelling its growth.
To tackle this issue, you'll want to bring your nitrates down to below 20ppm and reduce the CO2 levels. Increasing water movement around the bottom of the tank will also help. I'd also recommend cutting back on dry food, as uneaten food can contribute to the growth of this bacteria.
While there are some chemical treatments available, they're not a long-term solution unless you address the underlying nutrient imbalance. Most treatments involve antibiotics, which should be avoided in this case.
To tackle this issue, you'll want to bring your nitrates down to below 20ppm and reduce the CO2 levels. Increasing water movement around the bottom of the tank will also help. I'd also recommend cutting back on dry food, as uneaten food can contribute to the growth of this bacteria.
While there are some chemical treatments available, they're not a long-term solution unless you address the underlying nutrient imbalance. Most treatments involve antibiotics, which should be avoided in this case.
Re: Help with a struggling 125-gallon planted tank
Your plants don't need high light, CO2, or heavy fertilization. They're all easy plants, except for the blyxa. The fish you have, on the other hand, prefer low light. Cardinals are even light-sensitive. I think you're providing more light, CO2, and nutrients than your plants need, and the algae is taking advantage.
My tank, which has around 60 cardinals, cories, and pencilfish, is low light - it actually looks dim. I use Seachem comprehensive supplement at half the recommended dosage once a week. I also do an 80% water change every week. I've kept high-tech tanks before, but I've opted for low-tech now. High-tech means high maintenance, and I don't want that.
I use RO water, just like you, and my maintenance routine is simple. I do a 75-80% water change weekly, add minimal ferts, rinse filter sponges, and wipe the glass monthly. I trim and thin plants when needed, usually weekly. I clean the canister and pumps every three months. It doesn't take much time or effort, and my fish and plants are always healthy.
My approach to plants is simple: if it works with my treatment, it stays. If it doesn't, I grow something else. I've noticed that vals like hard water, but they take ages to grow. When they do, they go ballistic, and I end up removing them.
By the way, you'll see a big change in your dwarf chain loach's behavior if you increase the group size to at least 10. I suggest 20, but they're sensitive to nitrates, so you need to sort that out first. Your tetras will also be brighter and live longer if you get the nitrates under control. In my tank, I never have detectable nitrates before a water change. Aquatic plants don't need nitrates; they'll use ammonia instead. If you get the balance right, your plants will clear the ammonia without producing nitrates, and your filter will just be for mechanical filtration and water movement.
Just saw @coltin's post about dry foods. I've noticed that my tank gets cyano if I feed flakes, so I've stopped doing that.
My tank, which has around 60 cardinals, cories, and pencilfish, is low light - it actually looks dim. I use Seachem comprehensive supplement at half the recommended dosage once a week. I also do an 80% water change every week. I've kept high-tech tanks before, but I've opted for low-tech now. High-tech means high maintenance, and I don't want that.
I use RO water, just like you, and my maintenance routine is simple. I do a 75-80% water change weekly, add minimal ferts, rinse filter sponges, and wipe the glass monthly. I trim and thin plants when needed, usually weekly. I clean the canister and pumps every three months. It doesn't take much time or effort, and my fish and plants are always healthy.
My approach to plants is simple: if it works with my treatment, it stays. If it doesn't, I grow something else. I've noticed that vals like hard water, but they take ages to grow. When they do, they go ballistic, and I end up removing them.
By the way, you'll see a big change in your dwarf chain loach's behavior if you increase the group size to at least 10. I suggest 20, but they're sensitive to nitrates, so you need to sort that out first. Your tetras will also be brighter and live longer if you get the nitrates under control. In my tank, I never have detectable nitrates before a water change. Aquatic plants don't need nitrates; they'll use ammonia instead. If you get the balance right, your plants will clear the ammonia without producing nitrates, and your filter will just be for mechanical filtration and water movement.
Just saw @coltin's post about dry foods. I've noticed that my tank gets cyano if I feed flakes, so I've stopped doing that.
Re: Help with a struggling 125-gallon planted tank
I'm almost certain this isn't Cyanobacteria. I've dealt with that before and it's just not the same. The texture is completely different - it's not slimy at all and I can't rub it off the leaves. It's also only on the oldest plant leaves and doesn't seem to be spreading to anything else in the tank.coltin wrote: ↑Sat Mar 01, 2025 5:31 pm It's likely you're dealing with Blue Green Algae, also known as Cyanobacter bacteria - it's a photosynthetic bacteria that thrives on nutrients, low oxygen levels and red light. The high nitrates, fertiliser and CO2 in your tank are essentially fuelling its growth.
To tackle this issue, you'll want to bring your nitrates down to below 20ppm and reduce the CO2 levels. Increasing water movement around the bottom of the tank will also help. I'd also recommend cutting back on dry food, as uneaten food can contribute to the growth of this bacteria.
While there are some chemical treatments available, they're not a long-term solution unless you address the underlying nutrient imbalance. Most treatments involve antibiotics, which should be avoided in this case.
@coltin said the fertiliser and CO2 are encouraging the growth, but I'm not convinced it's Cyanobacteria. I've had that before and it coats everything in a blue-green slime. This is mostly brown with a bit of dark green. It's not spreading to the substrate, wood, or any other surfaces.
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