Who Gives A Fish Forums

Hey guys, setting up my 18L nano planted tank for my betta, been running fishless for almost 8 weeks now. My betta's currently outdoors in a 15L bucket with lots of hornworts and amazon frogbit. Although it's fishless with lots of plants, nitrate in the nano tank is always high (>80 ppm), while in the bucket outdoors it's steady at 10 ppm.

My nano tank setup is:
Tank: 18L with undergravel plate
Substrate: 4 layers - pumice, bacteria starter, base fertilizer, and soil
Hardscape: Red stone
Plants: cabomba, bucephalandra, staurogyne repens, amazon frogbit, hornwort
Filter: mini HOB with sponge and a small amount of seachem denitrate
Lighting: 3w led (30 cm blue-white) running 6 hours a day
Fertilizer: liquid carbon and low concentrate fertilizer

Here's how it looked 2 months ago when I planted the tank



And here's how it looks now



Most plants (except hornwort and amazon frogbit) are struggling to grow, and some are dying. Some bucephalandra melted in the first few weeks, but now they seem to be adjusting. Ammonia and nitrite are 0 ppm, but nitrate is always high after a few days. What's going wrong in this tank? Can you guys give me some advice?

I'm curious, have you worked with this substrate and fertilizers in other tanks before. It might be worth testing them out in a separate container for a week to see if they're leaching anything into the water.

Also, have you checked your tap water for nitrates. If your tap water already has high nitrates or your water company uses chloramine, that could be part of the issue.

I'm assuming you added the Seachem denitrate after you noticed the high nitrates. I'm not super familiar with the product, but I know some of those resin-type products are meant to be used for a short period, like 24-96 hours, and then removed or recharged. Over time, they can get saturated and start releasing their contents back into the tank.

This is my first time using these products in an aquascaping tank. Although I haven't used them before, they're quite popular and highly recommended here.

My tap water nitrate level is around 10 ppm.

I've used Seachem denitrate in another tank to lower nitrate levels, but unfortunately, it didn't work out. I think it's because the water flow wasn't precise as recommended. I believe it's functioning as a normal bio media, helping to reduce ammonia and nitrite. I added some to the HOB filter to aid in the cycling process. Seachem denitrate is essentially a smaller version of Seachem matrix (pumice), so I don't think it's the cause of the issue.

Could the high nitrate be due to the dying plants? And are the plants dying because of the high nitrate?