How should I estimate actual water volume to include displacement by substrate, plants, and decor? My tank is 30x12.4x22in. I have one more big plant (they are all fake) than pictured here. It's marketed as a 37 gal but idk how accurate that is most volume calculator put it at around 35 gals {}

What you want to do is to fill a measuring tub with water. Then put your decoration is the water and record how much the water level goes up. I don't know the formula off the top of my head, but I think this points you in a good direction.

empty the tank and the take a bucket fill it back up and remember how many buckets did u need to fill it back up and everytimes take same amount of water in the bucket. some buckets usually have it written how much water it can hold it will be in liters you can convert it to gallons through calculator. good luck!

@ASHLEY R COOK I was also curious. I used this awhile back. After entering the numbers, it turns out my 55 gal tank is closer to 48 gals. Not perfect but you'll get a better idea. Hope this helps.

Make a mark of your water level (painters tape) and take all decoration out. Make another mark of the new water level and measure from this mark to the previous water level. Call it delta H. Measure the inside length (L) and the inside width (W) of your tank. If you measure in inches use this formula to get the actual gallons of displaced water. L x W x delta H x 0.04329

I am wondering why you want to be that accurate but as a guesstimate I would assume 40% of your gravel bed consists of water. Use the same formula as above using the height of the gravel bed for H and multiply with 0.6 to get an idea of the displacement of the gravel.

In not necessarily wanting to be super accurate. I was just asking if I needed to account for substrate lol I just did my first PWC yesterday and I took the water level down to what looked like 50% but my ammonia went from 2ppm to .25ppm.(fishless cycle) So I was wondering why other ammonia dropped so low compared to the original measurement. It must be bc I didn't account for decor water displacement.

Usually it won't displace too much water, so stock for just under the amount you would do if your tank was empty! If you have a decent filter stocking as if your tank were empty is fine

Ahh, ok. Keep in mind that the test kits give you a range + error. I wouldn't be to concerned about numbers, the tendencies need to be in the right direction. My 2 cents.

My bigger piece of driftwood displaces 5 gallons of water. I have a 20g tall and I used it to clean the DW and it filled with 3 5g buckets. Not relevant to your question, but decor can displace a lot of water.