Too much hassle to return them to a shop?!? Sounds like a diamond bloke....
To be honest if I were you I'd immediately take them off his hands, sure ok 110l isn't the ideal size of tank for them but it'll be like an ocean to them in comparison to where they've been living currently and it would get them through till the New Year at least.
Once the New Year comes around you could then donate them to a local, reputable fish place (who may even give you a credit note for your trouble if the fish are well) or, if you've become attached to them you could look at upgrading the tank for them.
Overall, anything has got to be better than those poor fish spending any more time with that guy....or in the sewer.
I use a heater in my coldwater tank with fancy goldfish and would have to say I highly recommend it.
Up until the harsh winter of last Dec I didn't use a heater at all but found the fish became very lethargic and motionless for long periods when the water got very very cold and I don't think they reacted too well to the temperatures dropping down at night and then increasing during the day and back down again.
So I put a heater in, set it to the lowest setting of 19 C and the fish perked up within a couple of hours, when the temp drops (as is now the case again with winter setting in) the heater keeps a constant temperature.
I have seen this thread develop over a week or so and I simply have to say something.
I admire all of you who have taken the time out to provide help & advice for "Simplyobsessed" but unfortunately it seems pretty obvious they are not willing to act upon your advice which is a great shame.
Perhaps "Simplyobsessed" should reflect their namesake and become simply obsessed with reading up and researching on how to look after fish properly (information which plenty of members on here have kindly been providing links for) rather than being obsessed with ignoring top quality advice, carrying on regardless and then posting up messages a week later moaning about more fish they have needlessly killed.
Your readings suggest your tank isn't cycled (no ammonia should show up on the readings in a healthy cycled tank) so that explains the death of these fish.
Your tank is around 200 litres which would only be sufficient to house around 2 fancy goldfish, 3 at the very most provided you had excellent filtration and did regular water changes.
You already have 4 fancy goldfish in there and by trying to add another 2 more you are seriously overcrowding the tank. It may not look crowded but fancy goldfish heavily pollute their water so by adding so many fish their pollutants are less diluted in such low water volume hence the reason for the deaths of the two new fish.
For 6 fancy goldfish you are probably looking at a tank of around 500 litres to keep them healthy.
Also, having just an under gravel filter is insufficient, you would need an external canister filter really, I use an Eheim 2217 on my 200l tank and it's excellent.
Re: 550L Now Cycled! But how should i introduce my two Orandas?
Wow, 550l - that is my dream tank.
Out of curiosity what filtration are you using? I am planning (although it'll be quite a way down the line) to get a 500l tank for my Orandas to replace my 200l but I keep umming and arring over what filtration to use.
I use an Eheim 2217 on the 200l so my original idea was to purchase an Eheim 2260 and partner that with the Eheim 2217 on the basis that the 2260 can do the biological filtration and the 2217 I'll fill with sponge & wool to handle the mechanical filtration & polish the water.
However I am now seriously considering installing a sump when I get this huge tank as the added benefits of the additional water volume / being able to hide equipment in there / easier water changes etc make it seem so appealing, not to mention the possible ??? saved on having to buy an Eheim 2260 and extra power that would demand.
Sorry for the long message but as you've just setup a tank and keeping the same fish as myself I couldn't help but ask.
P.S Are you just keeping 2x Orandas in there? If so they are going to get massive, great work
Re: Stockings / Tights ... I mean filter bags.
If you wanted to use a large amount of Purigen (say for example if you have a lot of filters / tanks or simply need to put more than 100ml in at a time) then it may be worthwhile buying a larger quantity of Purigen and then bagging it up yourself.
I wouldn't use stockings or tights though, Purigen sell "The Bag" seperately but it's fairly pricey, a good alternative is this on ebay:-
Can't really help on the sump question, I thought those were primarily used for marine tanks but you may be able to use them with a tropical set-up I'm not sure, whether or not they would be better than using a canister filter is probably best answered by a marine expert.
You say it's a 5ft tank, do you know how may litres it holds? I would guess it's at least 250 - 300l, probably more and on that basis I would not recommend putting it on a TV stand or table as they are not designed to hold that kind of weight.
For a tank of this size you only have 2 options I think, either buy a dedicated aquarium stand (these are designed to handle the weight of aquariums) or build your own ensuring that you read up on it properly (loads of articles on the web) - I've seen a lot of people make their own stands / cabinets as it can save you a lot of ???, you can tailor make it to how you want it to look and hey, it's a nice bit of satisfaction knowing you built it yourself.
Also, whatever stand you decide to buy / or make please ensure that tank doesn't overlap the edges on any of the sides, even if just by a few cm as this will weaken the glass and over time will cause it to shatter.