I can only think your axolotl may have been stressed by a no. of factors:i.e.
1. Tank trying to cycle (see:
http://www.caudata.org/forum/messages/793/77098.html?1170154180) Cycling can take 3weeks to 10.5 weeks.
2. Removing/returning him weekly to do the complete waterchange. By doing this each week in a tank that is trying to cycle, it causes your tank to start cycling again (ie ammonia toxins have to start again to build up - this is normal in a cycling tank , but is better managed by doing partial waterchanges every day or every 2 days rather than 100% weekly - otherwise your tank will never get a chance to establish good bacteria)
3. If you had temperature problems, ie anything 22C+ then icecubes should be avoided as they cause fluctuating temperatures as they melt fast and the temperature can fluctuate from warm to cool to warm before you chuck in another cube. Also, doesn't help if its straight tapwater frozen (one of the aussie owners, a few years ago used the ice bottle method to cool his tank but had straight frozen tapwater - the bottles burst and his axie died)
4. Floating = may not have been gas, when stressed they start to float and usually helps if their water can be either lowered or if you have temperature problems, for axolotl to be put in the fridge in lowered water. The cold water slows its metabolism down, the lowered water is less stressful.
It's unfortunate that you didn't find this site a few weeks before he died, the above may have added to his stress and in turn illness. On the other hand he may very well have had some illness prior to you buying him and lack of good or any advice (from petshop) wouldn't have helped.
I honestly don't think you should beat yourself up about it, the majority of people that find their way to this part of the forum have had some problems very similar to yours or they also are trying to understand why their axie/s died or are sick.
Just a word of advice, one may not be enough
. But start with one, for a few months.
Before buying your axolotl, you should sort things out with your tank.
By that I mean, have you got the right size for one (or two if you think you might get a companion in the future). A 2ft/60cm/600mm long tank is a good size for one axolotl, decos, plants (if any) and a cave/flower pot/pipe/log/etc... Size of the tank is based on one adult axolotl being 30cm long (our largest are 32cm), + 30cm extra space for items and to move around. A 3ft long tank would be better for 2 axolotls.
Then substrate, bare, sand or permanent (by permanent I mean
www.caudata.org/forum/messages/793/77098.html?1170154180 - my sister did one in gravel, one with permanent sand, they look good, food doesn't get lost beneath the substrate, easy to feed and clean). Don't use river rocks or gravel - dirt gets trapped beneath and can cause water quality problems.
Buy a gravel cleaner/siphon if you don't have one - can be used with sand as well, and a few extra buckets if you don't have any, they come in handy for waterchanges etc..! I find the wider tube siphon works better than a skinny tube one (we have both) as sometimes poo/food gets halfway up the skinny one and falls down - can be damn frustrating as it disintegrates!
Buy a turkey baster, if you can find any (kitchen department part of supermarkets or other stores), they are handy little daily cleaners and saves you pulling out the gravel siphon daily.
I assume you have a thermometer!
If you don't have a filter, check out:
www.caudata.org/cc/articles/filters.shtml, we have filtered and unfiltered tanks. The unfiltered tanks get changed 2-3 times a week, the filtered ones once a week since they cycled.
If you're able to afford it I think you should also buy freshwater test kits for ammonia, nitrite and nitrate. You can buy a freshwater master test kit but can be a bit expensive but includes tests for all 3 as well as low and high pH (don't buy the all in one tests, they're confusing, buy the test tube type ones). These 3 are the most important ones to test when it comes to cycling, and sometimes it's just not convenient to take in tankwater samples to petshop to be tested every few days. What usually happens is when the petshop tests it they either, say its fine, or as one owner was told the water's poisonous and then try and sell you a product to fix the problem!
Once you've filled your tank with decos and tankwater, then I suggest cycling your tank before you get your axolotl. see:
http://www.caudata.org/cc/articles/cyclingEDK.shtml We used a small cube of raw meat, dropped it into the tankwater to start the cycling (it looked and smelled pretty disgusting after a few weeks).
The petshop will no doubt try and advise you that "this is all you need" to "cycle your tank" or "get rid of the ammonia" "fix the nitrite" and/or "fix your pH"! The only chemicals/things you need are the water ager and the freshwater test kits (if you intend buying them). Tankwater with water quality problems can be managed through frequent partial waterchanges.
While you're cycling your tank, you could think of possible food sources. Live food is good if you can find worms/slugs/slaters from a pesticide free location. If you don't have a compost bin, consider ordering some worms online, through a baitshop or other garden centre/bunnings (that sell worm farms) and start a worm farm. From reading some of the threads many aussie owners had trouble finding worms during the summer months, so if you started a farm at least you have a easy food source ready).
Don't use feeder fish, especially from petshop/market breeder, they carry parasites, nibble at gills/fin and other parts of axolotl. They can make your axolotl sick. Raw meat should be used only as treats. Frozen bloodworm should be thawed and rinsed before adding to tank. (I put ours in fine mesh net, let it melt, then rinse with tankwater/dechlorinated water before putting in the tank for our axie larvae) - this causes less fouling of the water. Don't use mealworms, and if you do, only occasionally, and chop the head off as they can damage insides of axolotls.
Apologies for the saga
. Look forward to hearing how you get on!
Axolotl Enthusiast