I don't have any experience w/neons and cardinals but I have read that those fish, in general, just aren't very helathy due to being highly inbred. If I were you and I had a community tank where only one or two types of fish were consistently dying, I'd stop buying those fish. Another nice, small schooling fish you could try instead of tetras are zebra danios. I've kept them before and found them easy, hardy, and attractive.

You didn't say what size your tank is or what volume of water you change routinely, but have you considered the possibility of warming the R/O water on the stove and checking the temp w/thermometer before adding it to the tank (or some other method by which you heat a portion of the water, then mix it w/remaining volume to appropriate temperature before adding to tank).

Unfortunately they are probably right, get the RO unit and heat the filtered water in a 5 gallon bucket with a good submersable heater.

Without a KH test anything you do lacks of value, so that 1/3 of RO to 2/3 of tap is like trying to cover the sun with a thumb.

PH is highly related to two parameters, General Hardness ( GH ) and Carbonate Hardness ( KH ). General harness is the meassure of all dissolved salts in the water, carbonate hardness is the measure of all dissolved calcium carbonate or salt that behave like calcium carbonate and that give the water it´s buffering prperties. High PH indicates high hardness. The trick is to reduce the hardness by eliminating the salts or by binding them to form chelates.

How to:

1.- Reverse Osmosis: it´s called reverse osmosis because it´s performed by pumping the water through a semi permeable membrane that traps pollutants and dissolved salts.

2.- Boiling: you boil the water and the boilibg process causes slats to precipitate.

3.- Ion exchange resins: the water runs through the resin and traps the undesired salts changing them from something else like sodium carbide.

4.- Acid: you add acid ( usually phosphoric acid ) or run the water through an acidic media ( like peat ) until you neutralize the buffering properties of the water.
But is not as easy as it seems, to play with the parameters you need to know what you are doing or you can kill the fish. Those are the most common methods but they are not perfect:

1.- RO, unit expense is high and after you have gathered enough water you still have to reconstitute the water to the proper physicochemical parameters, either by adding a certain ammount of tapwater whose PH, KH and GH values have to be tested prior to perform the mix.

2.- Boiling reduces the hardness but not all of it and in case the GH and KH are very high the benefits are minimal.

3.- Ion resin exchange, it´s easy to go to the pet store and purchase one of those water softener pillows, yes they will leave the water "soft" ( low general hardness ) but since they only exchange one salt for the other the KH remains the same, the water will be soft but still very alkaline. In order to make the water soft on both sides ( GH & KH ) you need another resin, problem is that those resins are not easily obtained and need a special unit.

4.- Acid: acid is a cheap but dangerous way to reduce the PH, problem is that until you reduce the bufferenig properties of water the PH will continue to bounce back, more acid is needed until you lower the PH again, a couple of hours later it bounces back and so on, it may take many days before you can finally maintain a stable PH reading.

The simplest method is purchasing RO water and slowly mix with tap water until you get the PH, GH and KH right. You will need a container to keep the water and a heater to heat it to get to the same temp found in your tank.

78 degrees is a little bit "cold" for tetras, 80° is what they should have.

Neons & Cardinals are soft water fish and I can understand
some of the problems you are having. A solution you might
want to look into is the use of African Cichlids or brackish
water fish with the parameters of your tap water.
I'm surprised your dealer didn't talk to you more about
the local water if it comes out of the tap at those numbers.

It's hard to fight nature. It would be much easier to have fish that can live with a high ph [cichlids]. Make sure to check the tap water [and a neighbors], not just the aquarium water. Hard to believe they didn't check ph sooner. Ph and ammo are the first things you should check.

Hello
I use collected rainwater and avoid all tap water for all my tetras. and planted tanks.. Have a 75 with Cardinals and Clown loaches setup in Feb of 01 abd still have 15 of the original 18 Cardinals and all 4 of the loaches.
If I run out of rainwater I use RO water from the grocery store. Works for me gary

RO units have come down in price. I have been using one for gee, almost 15 years. I have used ratios from 50 RO/50 tap to 90 RO /1 tap depending on what I was working with. Our water here sounds to be about the same. The best results I ever had with Cardnal Tetras was with a 75 ro/25 tap water.

Yes Bambi but, giving ratio RO/tap recommendations without knowing the specific GH and KH is not the appropiate way of doing it, for your area 75/25 is what works, but that doesn´t mean it will work everywhere.

In the above post it reads

"Our water here sounds to be about the same"

It is probably almost identical, that is the Ph of ours, and its probably carbonate hardness.

There seems to be very little risk, in going with a 50/50 or 75/25 mix, they will probably do better than they are now in either mixture. There is almost NO hardness where those fish come from, and the water is very acid. In the aquarium very soft acid water creates a unique set of problems. Uneaten foods, even the smallest bit breaks down very fast, and pollutes the water. Many bacterial diesases thrive in the acid enviornment.

I came on the 75/25 mix by a lot of trial and error, unfortunately I have seen thousands and thousands (I worked in the wholesale business for many years)of Cardnal tetras perish in our liquid rock here.

I stated:

"The best results I ever had with Cardnal Tetras was with a 75 ro/25 tap water."

Where in my post does it say do it? I simply offered up what worked for me.

Last but not least, they are in zone 5 like I am, if they are near the great lakes there is not a big difference in water even if it is well water vs lake water. I have collected native fish all over the region, and can tell you that with the exception or marshes, and some stagnant woodland ponds, the water throughout is pretty much the same, close enough. I have checked.

I'm just wondering what you might have in the tank as decorations or gravel. For instance, if you happen to have some coral or limestone or...? you could replace water and and add biphosphates all you want, but alkalinity would be continually leached into the water, keeping the pH high. Just a thought.

First, thanks for all the responses! In the past we had 4 tanks of various sizes going, but packed them away when we moved (gave our fish to a friend) and now are getting back into the hobby. Thought I knew more that I guess I did. We never tested the Ph in the past; obvioulsy it was okay, as one tank housed discus and cardinal tetras, and one was saltwater and we had no problems. We're thinking of doing discus again but would have to address this issue first. So, some answers to your responses: melle: our tank is 55 gallon, we do 15 or 20 gallon water changes. In addition to the neons and cardinals we have also lost 2 zebra danios and 4 rummy-nose tetras. Bambi: When I read your response, I went "duh"...such an obvious and workable solution! We still have our bucket from the old days (we use the Python now), and several heaters not currently in use. Thanks you! raul: You're teaching me there's much more to know and keep up on. Thanks for the input. fleur guy and James: Makes sense to go for a harder water fish, but unfortunately the only cichlids I really like are the discus. And I agree, James, I had assumed the store tested the Ph as part of their testing routine, was very disappointed they did not. We had to replace our old test kits when we set up this tank and the store person told us all we needed were the test kits for ammonia, nitrite and nitrate. Only when we kept killing our fish did they explain that the Ph level is so high here. garyfla: Rainwater sounds like a option, thanks. petiolaris: Nearly all the decorations in our tank were used in our tanks before; there is no coral but two of the large rocks may be limestone (white with black streaks). The only exception is the wood we bought from the fish store that has a base of glued-on gravel.
And now, if you're still with me, the good news is NO fatalities in the last 6 days! Even have 2 cardinals hanging in there and looking healthy. Thanks again.

I would conduct a series of experiments with the various pieces of decorations, as well as with your tap water and see what might be the root cause.