Having just done this and failed with a few aspects, and won in others, I thought someone might find some tips handy (heck, if I ever do this again, *I'll* find these tips handy!). 1) The Juwel instructions are not very helpful. Particularly not on the smaller tanks like a Rio 125. You will find that even after trimming to height, you *cannot* wedge a panel whose width has not been trimmed down somewhat into the tank; I found I had to trim off quite a wide strip from the edge that butts up to the filter to get it to fit into the tank, despite the instructions clearly telling you *not* to trim this panel. If I recall correctly, it was a width of about 3 square blocks. I think the height measurement they suggest is a bit short. 2) Don't assume the filter is actually straight and flush; mine seemed to be at a slight angle, and resulted in about an hour of slow, gradual trimming to get it as flush as I could. If a small gap here bothers you, you can try putting some silicone along the filter and pushing the background up against it (I didn't). This is probably less of a problem if your tank is big enough that you can leave the "lip" on the piece that fits against the filter. Also, the lighting in the tank will be so bright, you'll hardly notice the gap if it is small (but when you notice the white wall behind whilst fitting the thing without the tank lights on, it's certainly irksome! Some black paper, card or even electrical tape up the back of the tank along that gap will make it pretty close to invisible too). Also, when you're trimming to fit, don't make the push-fit so flush that you put strain on the glass; this just seems like a bad idea. Snug, yes, tight, probably not. 3) Speaking of the filter, Juwel amazingly don't take the outlet pipe into consideration, so you'll have to carve a small channel for it! I made mine just big enough that you could wedge-fit the background in behind it; fit the outlet tube and rubber gasket thing onto the filter outlet before you put the background in. 4) It's going to be much, much easier to work on the tank if you don't try doing this in place - work on the floor (so you can see the back of the tank behind the background) or a large table. 5) Watch out - if you measure the height and cut from the base of the tank to the crossbar (which seemed sensible to me) the silicone on the bottom (which wasn't particularly well finished IMO) *will* push the base of the background forward far enough that your silicone bead probably won't actually do anything on the bottom half of the panel (This happened to me :/). Make it a couple mm less than that height, or else cut a profile out of the back to account for this. 6) I found it helpful to put the tank on its side and put some weight (careful not to point-load or over load it and crack the glass!) in order to try and make sure that the background was pushed as flush to the back as possible - but my failure in step 5) meant only the top half of the background was effectively stuck down. You may find it even more helpful to push-fit the background (with silicone in place) first, upright, see how the silicone is doing, and then put it on it's side with weight on (I used 2x7 kg bags of flourite and a ~1kg lead weight). I briefly considered G clamps before dismissing this as a bad, bad idea (imagine if they drop on to the bottom...!). 7) If you only have a short stanley-style craft knife, get one of those longer snappy-blade type box cutter knives - the background is quite thick, and you'll get a neater result cutting all the way through in one go, instead of doing the back and then turning it over and cutting through the front 8) Cutting the front fake rock is very "crunchy", you're not going to get an absolutely perfect finish (although you could perhaps see if something like a Dremel tool with the right attachment works; I didn't think of trying). Also, there are occasional large void spaces in the foam which will probably collapse on you. I'd suggest being slightly generous in your measurements and then trimming, rather than having dirty great gaps. 9) The join is very, very obvious and shows considerable room for improvement; try and hide it with your aquascaping. 10) On the whole, I'm not really very impressed with the product, which retails for nearly R500 a pop, and the minimum you'll need is 2 (and it's shocking how little of the second panel you actually need in a Rio 125). If they improved their fitting instructions, sorted out the *terrible* join, and took into account the fittings on their own tank, it would be much easier to recommend it. If you don't like DIY, avoid! You could, if you wanted to, probably use the offcuts to cover the filter box - remember to trim holes in it for any holes/pipes in the filter box; they're there for a reason. Stupidly, I didn't take pictures of the whole process, and in any case, only had a crappy cellphone camera to hand. The background I used was 2x Stone Granite.