Need help with a struggling 125g planted tank

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Rossbug

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So I’ve had this tank set up since the winter and can’t seem to get things to go right. I’ve got fluval stratum substrate, 2 fluval 3.0 lights, an fx6 filter, and pressurized co2. I had been using thrive all in 1 fertilizer, but recently switched to the GLA dry ferts about 2 weeks ago. For stocking I have about 60 cardinal tetras, 25 phantom tetras, 1 SAE, and 3 dwarf chain loaches.

My plants grow bright new growth, but old growth quickly dies off. Old leaves start by getting covered in dark green/ brown gunk, and then start dying off from there. Nitrates are very high 60-80ppm, and are back high again within just a few days of a large 40-50% water change. I use a mix with mostly RO water as my tap water is extremely hard. I saw no improvement all when switching fertilizer. The plants I have currently are all relatively easy but nothing does well. S. Repens, limnophila aromatica, ludwigia repens, jungle val, Amazon sword, AR mini, and blyxa japonica.

Any advice would be appreciated. I Can’t figure out what I’m doing wrong. All my plants grow great in my 40g but start showing these deficiencies in the 125g despite plenty of ferts and high nitrate levels.
 
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Blue green algae (Cyanobacter bacteria). It's a photosynthetic bacteria that loves nutrients, low oxygen levels and red light. Your nitrates, the fertiliser and the CO2 are encouraging it.

Get your nitrates down to less than 20ppm, reduce the CO2 and increase water movement around the bottom of the tank. Cut back on dry food as well because uneaten dry food causes this stuff to grow.

There are a few chemicals out there to treat it but unless you reduce the nutrients, you won't get rid of it permanently. Also most treatments involve using antibiotics and they shouldn't be used for this sort of issue.
 
Aside from the blyxa none of your plants need high light, CO2 or heavy fertilsation as they are all considered easy plants.
Now for the fish, the fish you have prefer low light and in the case of the cardinals are actually light sensitive. I suspect you are providing far more light, CO2 and nutrients than the plants need, so the algae is taking advantage.
FWIW the tank in my signature has around 60 cardinals along with cories and pencilfish. It is low light (i.e. actually looks dim) and I use Seachem comprehensive supplement at half the recommended dosage once a week. I also do an 80% water change every week.
Also FWIW I have previously kept high tech tanks. High tech means high maintenance, in my previous 450l tank this meant 2-3 hours twice a week pruning, thinning and cleaning. This is (one of) the reason I have now opted for all low tech.

I also use RO (for the same reason as you) and my maintenance on 4 tanks looks like this
  • 75-80% water change - weekly
  • Minimal ferts - weekly
  • Rinse filter sponges - weekly
  • Wipe glass - monthly or if needed
  • Trim / thin plants when needed - usually weekly (yes they still grow ;) )
  • Clean canister / pumps - 3 monthly
This takes almost no time or effort on my part and my fish and plants always look healthy. For plants my approach is if it works with my treatment it stays, if it doesn't I'll grow something else.

On a separate note you will be amazed at the change in behaviour / activity of the dwarf chain loach if you up the group size to at least 10 - I actually suggest 20, but they are sensitive to nitrates so you do need to get this sorted first. The tetras will also be much brighter and live longer lives if you get this to acceptable levels. In my tank I never have detectable nitrates before a water change. Aquatic plants don't need nitrates, they will use ammonia in preference and if you get the balance right will clear the ammonia without producing nitrate, and your filter is pretty much for mechanical filtration and moving the water around.

Edit: Just seen @Colin_T post re dry foods. One of my tanks gets cyano if I feed flakes, so I don't.
Also remembered that vals like hard water. I can grow them in my tanks, but they take ages to grow and then really take off. They do gather a lot of algae while waiting to grow and when they do start they go ballistic - so I have removed them.
 
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Blue green algae (Cyanobacter bacteria). It's a photosynthetic bacteria that loves nutrients, low oxygen levels and red light. Your nitrates, the fertiliser and the CO2 are encouraging it.

Get your nitrates down to less than 20ppm, reduce the CO2 and increase water movement around the bottom of the tank. Cut back on dry food as well because uneaten dry food causes this stuff to grow.

There are a few chemicals out there to treat it but unless you reduce the nutrients, you won't get rid of it permanently. Also most treatments involve using antibiotics and they shouldn't be used for this sort of issue.
99% sure this is not Cyanobacteria. I’ve dealt with that before. This is not slimey at all, and does not appear on any surface other than plant leaves. It also seems more etched into the leaves. Like I am unable to rub it off at all.
Blue green algae (Cyanobacter bacteria). It's a photosynthetic bacteria that loves nutrients, low oxygen levels and red light. Your nitrates, the fertiliser and the CO2 are encouraging it.

Get your nitrates down to less than 20ppm, reduce the CO2 and increase water movement around the bottom of the tank. Cut back on dry food as well because uneaten dry food causes this stuff to grow.

There are a few chemicals out there to treat it but unless you reduce the nutrients, you won't get rid of it permanently. Also most treatments involve using antibiotics and they shouldn't be used for this sort of issu
Blue green algae (Cyanobacter bacteria). It's a photosynthetic bacteria that loves nutrients, low oxygen levels and red light. Your nitrates, the fertiliser and the CO2 are encouraging it.

Get your nitrates down to less than 20ppm, reduce the CO2 and increase water movement around the bottom of the tank. Cut back on dry food as well because uneaten dry food causes this stuff to grow.

There are a few chemicals out there to treat it but unless you reduce the nutrients, you won't get rid of it permanently. Also most treatments involve using antibiotics and they shouldn't be used for this sort of issue.
I don’t know that it’s Cyanobacteria. I’ve dealt With that before and it’s very slimey and coats everything. Also very blue green. This is nothing like that. Mostly brown with a bit of dark green. Only on oldest plant leaves. Doesn’t spread to anything else in the tank like substrate, wood, etc.
 

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