"Can I use Miracle grow on them by diluting it in water and then watering it into the soil?"

Yes

" I had some house plants before and I use it and it gradually killed it. What is the reason?" It wasn't the fertilizer that killed the plants unless it was misused and/or simply acted as a contributor to another issue that was severely limiting by it's own lack of merit. Nutritional issues often act in concert with poor soils and over-watering to force the plant to deal with limitations the plant is not genetically programmed to tolerate, which produces stress levels so severe the plant's systems just start to wobble like an out of balance top until they simply cannot function as designed.

Fertilizers with 3:1:2 ratios are good for houseplants because they furnish nutrients in the ratio at which the plants actually USE the nutrients. This allows you to keep fertility as low as possible w/o having to deal with deficiencies, which is a big plus for several reasons. Miracle-Grow comes in many RATIOS. 30-10-10 is a 3:1:1 ratio. 24-8-16 and 12-4-8 are both 3:1:2 ratios and 20-20-20 is a 1:1:1 ratio. If I had to choose a MG product as my "go to" fertilizer for almost everything I grow, I'd probably make it the liquid 12-4-8 mixture because of how easy the liquid is to use, compared to the granular. If my choices were broadened so as to include Dyna-Gro's Foliage-Pro 9-3-6, I'd definitely opt for the 9-3-6, another 3:1:2 ratio fertilizer. I like it because of its ratio, because it has all nutrients essential to normal growth (note that your MG lacks Ca, Mg (secondary macronutrients) and several micro-nutrients. FP 9-3-6 also gets most of its N from nitrate sources, which helps keep plants from getting leggy and coarse under low light (indoor) conditions, an important consideration for houseplants.

Growing well is a rather holistic venture that must be looked at in its entirety. In most cases, a grower's growing experience comes apart at the seams because the grower finds himself unable to keep the plant's root system healthy and functioning normally. My experience has been that if you focus on keeping roots happy and succeed, the rest suddenly becomes remarkably easier. Let me know if you want to chase that thought a bit to see if what I said makes good sense.

Al

Only feed a plant that is growing healthily. And most new bought plants have fertiliser in the soil already so shouldn't need feeding. Plants are more likely to die from overfeeding than not feeding.

"Only feed [fertilize] a plant that is growing healthily."

In a good % of cases that's true, but the instruction paints with a brush too broad. We need to be a little more discerning about what care practices we put in place. To wit, the advice would condemn all plants suffering from a nutritional deficiency to death, simply because we need to add the deficient nutrient (feed [fertilize]) in order to reverse the decline. In many instances, it's a nutritional deficiency that stays a plant from 'growing healthily'. A plant in decline because it's suffering from a nutritional deficiency can't recover unless its fertilized.

Al

But a plant bought from a nursery shouldn't be suffering from a nutritional deficiency. If that was the case I would pass on it.

You would THINK that plants bought from a greenhouse or nursery would have been fertilized recently, but we shouldn't really make the leap and assume it to be true, because a plant in it's travels and overstays from the point it was last fertilized to your home might be measured in days or weeks to months ..... and many nutritional deficiencies are mild or not readily apparent to an untrained eye. It's just a better practice to avoid sweeping statements unless they're dressed in some qualification.

Al

Well here at least the plants are potted up with fresh compost which itself contains nutrients, and often it contains little yellow fertilizer balls. Not sure of the situation in the US

What we call finished compost is extremely low in nutrients - so low, that a starter charge of fertilizer mixed into soils made with notable volumes of compost would be appropriate before anything is planted in it, as would be the alternate of fertilizing at the time the planting is established. We really have no idea how long it's been since the plant has been fertilized, so if the plant isn't exhibiting good vitality, the reason could well be a lack of adequate nutritiuon; so the point is, the advise to "only feed a plant that is growing healthily" is too broad to be appropriate. This, again, because a good % of those plants that aren't growing robustly are probably suffering from a deficiency of one or more nutrients. How do we normally fix nutrient deficiencies? We fertilize. What happens if we don't fix the deficiency? The plant continues to decline and dies. Just because advice like "Never fertilize a sick plant" (roughly the equivalent of what you said) is widely repeated doesn't mean it's sound. Broad statements almost always require qualification. If you still consider disagreeing, ask yourself, "What DO we do about plants that are not exhibiting robust growth because they are suffering nutrient deficiencies if we can't/shouldn't fertilize them?"

Al