Ok, let me see if I've got this right. So you had an established 64 litre tank, you bought the new filter and put it on that tank, then 3 weeks later you got the new 135 litre tank and transferred everything over, so this new tank has been running now for about 5 weeks. Correct so far?
When you upgraded the filter on the 64 litre, did you take the old filter media and put it in the new one to 'seed' it, or were you just running both side-by-side? What about when you started up the 135?
Did you transfer anything else from the 64 to the 135, like gravel, decorations, etc?
I suspect what is happening here is that you are seeing a bunch of mini-cycles. I don't think additional circulation will make a whole lot of difference, just time.
Ok, I just took a few minutes (I should have done this before) and looked up the TetraTec EX700. Is this what you have?
https://www.tetra-service.net/tetra/go/EB6E4F6D15C55F9A8D229E4E478584FF/?lang_id=2
These filters are identical to the Marineland C-series filters that are sold here in the states, which is what I use on my 55 gallon tank. I even watched the video, the only difference is the color of the tubing to and from the canister.
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This is a very good filter design, I've been happy with it. That being said, I am trying to figure out the comparable unit between the C series and the EX700, but I think it's the C-360. The Marineland version has models 160, 200, 360, and 530. The Tetratec models are 400, 600, 700, and 1200.
What are the dimensions of the filter media that you use? I'm talking about the black coarse pads for the bottom tray or the white polish pad in the top tray. I'll compare them to mine.
If it's the same size, I use that model on my 55 gallon planted tank, and I figured (on paper, at least) that I get about 3-4 turns per hour. So something isn't adding up...somewhere...
PS here's what my C-360 looks like
Is yours about the same size?