I may have a cheap and effective answer for you, a filter you can build for yourself for dirt cheap, under $30, and really more like under $20. A Molecular Absorption filter (kind of like carbon, but broader in capability) using PolyFilter media.
I can't swear to you this will work 100%. So if you try this and kill your octo, please don't kill me!
:P Just because my octo and other critters are very happy with this system does NOT mean yours will be...you may have stuff in your water this system can't remove. I've had great difficulty finding precise, comprehensive information on exactly what this system removes and what it doesn't, and how often it needs recharging. There's no question it removes a LOT of stuff, but it's also pretty clear it doesn't remove as much as a good (expensive) combination RO/DI system.
That said...here's what I did. Using some DIY information culled from the web, I assembled a filter using PolyBioMarine's PolyFilter material. PolyFilter is pretty neat. Many aquarists swear by it...it's great to have on hand to throw into a tank for emergencies to suck up bad stuff, especially in emergencies when something toxic in the water is suspected. My current "fire drill" plan is to toss one of these in the tank, do a major water change, and add carbon to my canister filter...prolly in that order.
Below I have listed links to all the information I could cull about this media. It's very simple...cut the pipe, drill the holes, roll up the media and insert it, and you're done. I run my water through a faucet-mounted PUR filter (which is basically a carbon filter with some mechanical filtration too) before I put it into this filter. I discovered some problems while doing this...my filter was a bit awkward with the sinks I have access to. Since you'd probably buy a large amount of cheap PVC pipe anyway, I might suggest you construct a "prototype" out of the pipe first and see if you can perfect your design so it's easy to fit into a sink while still allowing the vinyl tubing room to drain properly into a bucket. It also took a few uses before the rolled-up media fully expanded to fill all the gaps in the tube.
With this simple do-it-yourself PVC tap water filter you can achieve about the same results as with a more costly commerical TWP (Tap Water Purifier).
Cut a desired length of 2 & 1/2 inch PVC pipe.
Drill a hole in the center of the end cap, of a suitable size to match the vinyl hose diameter for insertion or attachment.
Attach the end cap to one end of the PVC pipe.
Attach a desired length of the vinyl hose to the drilled hole in the end cap.
Roll up a piece of Poly-Bio-Marine Inc.'s Poly Filter material and place it inside the PVC pipe and add the carbon.
Simply pour or run tap water down through the open end of the PVC filter and direct the treated water that comes out of the vinyl hose to your tank or storage container.
100% silicone sealant can be used to seal the vinyl hose and end cap connection.
Here's the main reason I ascribe some faith in this filter...a review of a commercial water filter that is made using this media. It appears it's often used in hospitals. It seems impossible to find now, but it's easy enough to copy using the above instructions! Note...this media comes in two forms, the sheet form that's discussed in the above directions, and a disc form which the commercial filter is designed for. Some suggestions are that the discs are stronger than the sheets...so if you want to try those, I don't think it can hurt.
A review of the media itself:
Another reference to the use of it as a filter (he's talking about it as part of a really complicated system, but it's good on its own as well.)
If you have bad water in your area, I'd be really nervous about using this, since I can't be sure exactly what it can and can't remove. Otherwise...it might be risky, but it's worked for me and others, and the price is undeniably nice.