With a good setup you shouldn't need to add any chemicals at all, if you have to add chemicals to control the water quality then there's something wrong with your setup.

With the correct size tank, adequate water volume and filter, and some live plants, you should have an ecosystem in miniature that maintains itself.

I don't use any chemicals at all, apart from Reptisafe if I run out of clean rainwater and have to use tapwater to top up my tanks.

Do you buy most of your kits on-line or in retail shops?

If the ecosystem is maintained as suggested then surely we don't need the testing kits...as it will only show negative results. So this is peace of mind rather than a neccesity, right?

I find that on-line prices are not really that cheap. At times they may even be more expensive and plus postage costs.

I only use plastic plants because terrapins will eat the real plants and may clot the internal filter.

I'm not going to be able to afford a proper lighting sets for the terrapins.

As an alternative, I'm going to go to the gardens centre and buy a cheap old fashion light bulb with red glass...similar in 'style' to this one below

PetClub UK - Zoo Med Nightlight Red Reptile Bulb 15W

My question is...60W or 100W? It will be placed approx. 30cm above the basking area.

Does red light bulb transfers more heat than a colourless one?

I buy a lot of my stuff from Surrey Pet Supplies, they are one of the cheapest and just a couple of miles away, because my tanks are largely self regulating I don't test my water very often, and when I do the levels are always very low which shows the system is working.

It's still important to do tests though just to check it's all working OK, filers need cleaning from time to time and a slow rise in chemical levels can be an indicator of this.

Another reason why internal filters are pretty useless! A big external filter won't clog as easily or quickly as a little internal does, internal filters really are no good for turts.

And turts need to eat plants, veg is very good for them. So you just keep adding more plants as they get eaten, or else you have a smaller tank alongside the main one and grow the plants in that, pump the water from the filter into that tank first then back into the turt tank. That way the plants get to do their job of removing nitrates and they don't all get eaten, as the plants grow you can feed the excess to the turts.

What do you mean by a "proper" lighting set?

Red lamps are not neccessary, a clear lamp is better as they respond to the high light levels, all you need is an ordinary spotlamp 40-60W, they cost £1.49 for three in Wickes.

You already have a UVB, what % output is it?

The one designed for terrapins...which is expensive...reflector...stand etc. Spot Bulb Holders, Reflectors & Guards

I get a 60W clear light bulb from asda then.

Repti glo 5.0uvb (emits 30%UVA and 5%UVB)...20W one...
REPTI GLO FLUORESCENT BULBS 2.0 SERIES, Reptile Lighting, Blagdon Reptile House

You don't need all that stuff anyway, a spotlamp doesn't need a reflector because the bulb itself is a reflector. I use a Clamp Lamp to mount mine, for just over a tenner it's a safe and easy way to mount the spotlamp, it provides some protection for the bulb so you don't accidentally break it or burn yourself on it, and comes pre-wired with a plug ready to use.

The wattage of the bulb you use will depend on the temperature, I use 40W spots and mount them about 8-9" above the basking area, higher wattage bulbs obviously need to be mounted further away.

Your UVB is OK but 5% is the absolute minimum, when it needs replacing after 6 months I suggest you get an 8-10% one, I prefer the Exo Terra Repti Glo 10.0 Compact, which concentrates the light over the basking area where it does the most good.