High nitrate in new planted tank

Hello guys, I'm setting 18L nano planted tank for my betta and it's running fishless almost 8 weeks now. My betta currently stay outdoor in 15L bucket with lot's of hornworts and amazon frogbit. Although it's fishless and have lot of plants but nitrate in the nano tank is always high (>80 ppm), while in the bucket outdoor steady at 10 ppm.

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

This is how it looks when I was planted the tank two months ago

This is how it looks now


All plants (except hornwort and amazon frogbit) have difficulty growing and some die too. Some bucephalandra melting in the first few weeks but now seems to be adjusting. Ammonia and nitrite is 0 ppm but nitrate always high after few days. What is going wrong in this thank? Can you guys give me some advice?

Have you used the substrate and fertilizers before in other tanks? I would do a test on each (put some into a 1L container for a week and test the water) to see if they are leeching anything into the water column.

Have you tested your tap water? If you have nitrate in your tap water, OR your water company uses chloramine, that could both explain the high nitrate.

I assume you added the Seachem denitrate after you noticed the high nitrates? I'm not familiar with the product but many of those resin type of things are meant to be used for 24-96h and then removed or recharged since they become saturated over time and may leech their contents back into the tank.

This is my first aquascaping tank so I haven't used them before. But this product is commonly used here and I found it quite recommended.
My tap water nitrate is around 10 ppm.
I've been using seachem denitrate in other tank trying to reduce nitrate but have no luck with it, probably because the water flow is not precise as recommendation. So I believe it's working as normal bio media that reduce ammonia & nitrite. I use some of it in the HOB filter to help boost the cycle. Denitrate is just a small version of seachem matrix (pumice) and it's not leeching.
Is it possible I got high nitrate because of dying plants? And why the plants is dying?