Okay 8 coffee filters later the filter looks clean but the water is still stained black... And I would recommend doubling up or even tripling up on a slurry test because the amount of water lost in the filter... I used a half cup of distilled water in my slurry test 2 a quarter cup of soil and that's all the water I had left. My blue-collar scientific mind is screaming at me that this is not that accurate or really feasible when growing in pots because you don't want to sacrifice that much soil. The pH of the water drop to 6.49 and there's a picture of the TDS meter

You can check your fertilizer/tea mix beforehand and then test if it's building up or leeching out. It's not so important what the number actually is with organics, but how it's chancing. So if you make a tea and it has a EC of 500, but two weeks later you do a slurry/pour through that shows 1000 then you know you’re building up nutrients, and want to cut back. I notice some nutrients (like fish emulsion) will show an EC, where other stuff won't. (If I remember right kelp doesn't). So you'll just have to do some experimenting and make some educated guess.