signs of low gh

Don't get frustrated, it is just that this could be so many things. I will just shot gun it for you as it comes to mind. Low GH could come in the form of low calcium, which produces small shriveled leaves, or low magnesium, which produces very white and easily infected growth. What I mean by easily infected is it is taken over quickly by algae like GSA, BBA etc.

If your plant leaves are falling off at the root, i'm assuming it's not a stem plant first of all. Maybe a Val or crypt? Again, you would have to be more specific in your question.

Having something melt at the root could be because you berried the rhizome into the soil or substrate. It could be because you moved the plant, and now the roots cannot support all of the leaves it could previously.

Ruling out all of the above, if it is a root feeding plant, pick up some root tabs from your LFS and try them out. I have had jungle Val show a similar problem as what you're describing. I tried moving the plant to different parts of the tank to give it different light. My other plants were doing just fine, but this one wasn't really growing and would lose leaves at the base. Then I put down a root tab directly under it and it started giving me rapid new growth.

Remember that certain plants, like jungle val, grows from base up. If a leaf is melting at the stem, it can mean a lack of nutrition for the new growth. This would produce a weak spot as you describe. If the rest of the leaf is green, then all that means is while that portion was growing, the plant had sufficient nutrition.

What sort of light do you have?
I have had quite a few plants fall apart especially at the base, but all over, too. Poor light.
Maybe best to answer a bunch of questions:

Describe the type of lighting, watts, what kind of bulb, what kind of fixture, hours on. If it is a specialty fixture and you can provide a link that may help.

What kind of substrate?

Describe your fertilizer schedule:
Product, dosing.

Are you adding any form of carbon? (yeast/DIY, Excel, Pressurized)

How often are you doing water changes? Are you using straight tap water? RO? Describe how you prepare the water for a water change. What products do you use?
Please run all the tests you have on some straight tap water, and, if you prepare the water specially for the aquarium, test some that is ready to use.
If you do not have very many tests some of this information is available from your water company in their annual water quality report. (Unless you are on a private well)

Test the aquarium with all the tests you have right before a water change. If you do not have very many tests, some stores will test the water for free or cheap. Write down the actual numbers and the units. Do not just nod when they say, "It all looks OK".
Ammonia (ppm)
Nitrite (ppm)
Nitrate (ppm)
GH (ppm or German degrees of hardness)
KH (ppm or German degrees of hardness)
pH. Please test pH of tap water several times: Right out of the tap, then allow a glass of water to sit out for 24-48 hours and test again.

I'm using 8 10000k t5 bulbs 39w for 9 hrs a day on a 180 gallon tank
My substrate is eco complete
Fertz I've recently started dosing 2 tsp kno3
1.5 tsp kh2p4
50ml of trace and
12 ml of Fe every other day and never macros and micros on same day
Dyi co2 1 bp 3 seconds
40% water changes weekly
Ph in tank is 6.6 6.4 out of tap is 7.0
Ammonia 0
Nitrite 0
Nitrate 0
Don't have a gh or kh test kit my lfs didn't have any more and didn't get a chance to go somewhere else
Plants look green and very healthy except the fall apart at the roots real quick I took the advice of the op and got root tabs hopefully it helps also I just planted glosso but they seem to be melting and not really taking off I can't understand it what am I doing wrong this is my first planted tank so I'm not so experienced any help is certainly appreciated