ANTX-a (Anatoxin-a) can be removed via ozone filtration and/or reverse osmosis. See citation below. I was not able to easily locate a full text, but I imagine that if you ask the corresponding author very nicely, they'll send you a copy of the paper, and might even give you some practical advice. Keep in mind that if you are having ANTX-a problems in your local waterbody, you're probably having cyano-toxin problems in general; there are likely other cyano-toxin present in your water at relevant concentrations.
Granulated activated carbon may also remove ANTX-a, but how exactly this works (i.e. the most effective porosity and whether biodegradation occurs after adsorption) is an active area of research at this time.
If the water authority is allowing you to drink the water, it's likely safe for your fish. ANTX-a is an acetylcholine mimic; both humans and rays have acetylcholine receptors. Given that the cyanobacterial species associated with environmental contamination with ANTX-a have a world-wide distribution, it's likely that rays have encountered this compound at some point in their evolutionary history, and therefore possess adaptations to deal with background levels of contamination. That is, I wouldn't stress out about it if you have "some" ANTX-a in your tanks. With the amount of money you have locked up in rays and the water authority testing on a weekly basis while you are doing water changes 2x/week, I think it would be prudent to install a RO or ozone pre-filter.
This is unlikely to have caused they death of your ray by itself given that you have not reported ill-effects in your other fish, but this is certainly a source of that amorphous and cumulative thing we call "stress".
++++++++++++
Removal of the cyanotoxin anatoxin-a by drinking water treatment processes: a review
By:
Vlad, S (Vlad, Silvia)
[ 1 ] ;
Anderson, WB (Anderson, William B.)
[ 1 ] ;
Peldszus, S (Peldszus, Sigrid)
[ 1 ] ;
Huck, PM (Huck, Peter M.)
[ 1 ]
JOURNAL OF WATER AND HEALTH
Volume: 12
Issue: 4
Pages: 601-617
DOI: 10.2166/wh.2014.018
Published: 2014
View Journal Information
Abstract
Anatoxin-a (ANTX-a) is a potent alkaloid neurotoxin, produced by several species of cyanobacteria and detected throughout the world. The presence of cyanotoxins, including ANTX-a, in drinking water sources is a potential risk to public health. This article presents a thorough examination of the cumulative body of research on the use of drinking water treatment technologies for extracellular ANTX-a removal, focusing on providing an analysis of the specific operating parameters required for effective treatment and on compiling a series of best-practice recommendations for owners and operators of systems impacted by this cyanotoxin. Of the oxidants used in drinking water treatment, chlorine-based processes (chlorine, chloramines and chlorine dioxide) have been shown to be ineffective for ANTX-a treatment, while ozone, advanced oxidation processes and permanganate can be successful. High-pressure membrane filtration (nanofiltration and reverse osmosis) is likely effective, while adsorption and biofiltration may be effective but further investigation into the implementation of these processes is necessary. Given the lack of full-scale verification, a multiple-barrier approach is recommended, employing a combination of chemical and non-chemical processes.
Author Information
Reprint Address: Vlad, S (reprint author)
Univ Waterloo, Dept Civil & Environm Engn, 200 Univ Ave W, Waterloo, ON N2L 3G1, Canada.
Addresses:
[ 1 ] Univ Waterloo, Dept Civil & Environm Engn, Waterloo, ON N2L 3G1, Canada
E-mail Addresses:
svlad@uwaterloo.ca
Dieselhybrid
this is now a sticky. Sorry for your loss but thank you for turning that into a contribution for your fellow fish keepers. Two thumbs up
Grinch
. Our water has tested back within the acceptable range so I have the drips back up in running from the lake. The water goes through a sediment filtration and UV but that's all before the tap. I should have mentioned this but I have a large RO system that feeds into 2 55gallon barrels daisy chained together with a float switch, still up from my saltwater days. Comes in useful so I never took it down. It's fantastic to learn that this is effective at treatment, as I'll admit I did use some blended in when I've been short on water the last few weeks. 
