What could be going on? Water is pooling behind skimmer plus skimmer not filling up after I put in additional filters in waterfall bio filter

[salpatel]

Hello all. Newbie here. I estimate my pond is about 1300 to 1800 gallons (it was built by a prior owner so don't have exact specs). It runs on a tsurumi vancs 50pn2.75s. I have some pictures attached showing the skimmer and the water pooling up behind the skimmer.

The pond was running fine without any leaking or pooling since I turned it on in early May, so I don't think the pump size is the issue. A lot of algae was forming in June so I decided to put more filtration. I have a thing that looks like a big aquascape waterfall filter (call it "biofilter") above the pond which falls into a mini pond that collects the water and forms two streams down into the main pond. The biofalls appears to be a previous version of the aquascape biofalls (https://www.aquascapeinc.com/biofalls-filters), but it could be a different brand because mine is not shaped lie a semi-circle, but more of a trapezoid. The biofilter contained some coarse mats (3) and a bag of lava rocks. I added two Matala black pads, two Matala green pads, a Matala blue pad, some bio balls, some barley bundles. I took the heavy netted bag of lava rocks out of the biofalls feature and put it in the minipond instead of the biofilter because I didn't want it to bend the matala filters and block up what is the "swirl zone" at the bottom of the "biofalls" where the water comes in and then fills up through the filtering material and flows out the waterfall. Also, I thought putting in the lava rocks in the top mini-collecting pond would just provide the same type of filtration as it was inside the biofilter, but make the topmini pond more like a bog with the lava rocks, pea gravel and some water plants. More filtration can't hurt right?

Okay, so after doing all of the above, I turn on my pond and the following starts happening. Note that this same issue happened in prior years but somehow fixed itself over the winter.

1. Pond is full, filtration set, everything connected properly, turn on.
2. Waterfall runs fine, water begins to drain out of skimmer, float ball valve lowers and new water flows into skimmer
3. Water continues to flow into skimmer to fill pond, but skimmer does not fill up completely for ball valve to stop water flow.
4. Instead, water pools behind skimmer and pond does not fill to correct height so less water goes to waterfall feature and waterfall stops working.
5. Meanwhile, the water keeps falling into skimmer and instead of filling up pond completely, water pools up behind the skimmer.

I looked for leaks behind the skimmer but could not find a location. The liner is above the leak near the area where the water is pooling as shown in the one picture. I believe the pump is the right size because it was operating 24/7 for a couple of weeks without this issue.