Do Tropical Fish Benefit From Live Plants

That's how I started with fish, my sons won some goldfish at the fair. And they were in a 25 litre tank at the beginning, moving to 60 litres a couple of months later  The fair is at the end of October; by the following summer they hardly fit in the tank and we gave them away to someone with a pond. They do grow very quickly as I've seen so make sure you get them in the pond as soon as temperatures permit. Then you can keep a betta in the 30 litre (with a heater of course)  .

Back to the pH problem. Can you list everything that is in both tanks ( though just fish and plants will do for those). Substrate and decor are the things we need to know to see what exactly is causing the pH difference. Since the tapwater that's stood 24 hours and the 30 litre are the same, what is in the 60 litre that's doing it?

What are you using to test the pH - make and strips/liquid/tablets?
What is the lowest pH level on the tester's scale? If 6.0 is the lowest colour, the actual pH could be even lower.

I use the API pH tester too; your pH could be below 6.0 as if it drops off the bottom of the scale it will continue to show the colour for the lowest figure on the scale.

Curiouser and curiouser 

So the only difference is the live plants or lack of them. OK, next silly question (though you might already have told us, apologies if you have) - what is the nitrate level in both tanks? And your tapwater nitrate as some regions have a tap nitrate pushing the legal max of 50ppm and that needs to be taken into account. Live plants do use nitrate as fertilser, and they use ammonia which stops the nitrate getting high in the first place. Nitrate is acidic and very high levels can lower the pH.

The nitrite and ammonia could be affecting the pH in the tropical tank. Nitrite is acidic and ammonia is basic so they pull the pH in opposite directions depending on the proportions of each one. But they are both bad for fish. At your pH, the ammonia reading won't be a problem - ammonia exists in two forms in water, toxic ammonia and less toxic ammonium. The lower the pH the more that is in the less toxic form. But nitrite is more toxic at low pH than high pH so you really need to get that down below 0.25, preferably zero, by extra water changes. Once both ammonia and nitrite stay at zero, you might well find you pH sorts itself out.

To get them down you need to do a water change as soon as you see a reading above zero, and as big as necessary to get them down to zero. 25% a week is for when ammonia and nitrite are at zero, and is to remove nitrate and other chemicals which have built up, and to replenish minerals etc which have been used up.
At your pH, ammonia won't be a worry unless it gets quite high, but nitrite is a worry.

Adding bacteria is not really an option as most of those bottles don't work. The vast majority contain the wrong species of nitrite eating bacteria and some contain the wrong ammonia eaters too. The only one easily available in the UK that does contain the right species of both bacteria is Tetra Safe Start, but even that doesn't work if it has been stored incorrectly at any time since manufacture.

hi sue, update on my tank, i got nitrite down zero and nitrate to about 1 ( yellow) ammonia about .25, after a week should i be getting a low reading on nitrites and nitrates as i more or less get nil on the readings ,i thought i might get some sort of reading i.e. .25 or something like that.. just want to make sure i have bacteria and its working,

well ill keep plugging away with it, its very rewarding when you manage to get the water in good condition though can be quite time consuming  :-) thanks

How cold is cold? With the two common platy types (and most in the shops have both species in their ancestry) Seriously Fish recommends a minimum temp of 20^oC. If your tank is colder than this, I would suspect they are too cold. A few years ago, I had two bolivian rams in my quarantine tank and unknown to me, I had managed to crack the heater. I finally noticed the water was cold. The fish were behaving exactly the same way you describe the platies.

What are the other fish in with them? They might cope with low temps better.

I've been running my heated tropical tank at 21/22c with no I'll effects for the fish so 20 should be easily warm enough for 'temperate' fish. It does sound like something else is likely to be the cause given that your other fish are fine.