How should one handle a situation where his water tank is filled with gecko droppings? Should he throw all the water out and not use it for ablution and obligatory bath?
If the water in the water tank is a little or less than two qullahs  then the water is considered as najis, even when the droppings didn’t change the smell, colour or taste of the water.
If the water is two qullahs or more, then the droppings in the water don’t affect the purity of the water on the condition that it doesn’t change any of the three main characteristics of the water. 
However, if the water is two qullahs or more (large volume), but the droppings changed the smell or taste or colour of the water, then the water is considered as najis. This is the opinion presented by Imam al-Nawawi.
He cited the opinion agreed by Syafi’iyyah scholars who stated that a lot or a little, flowing or stagnant, if the najis in the water changes any of the characteristics of water in terms of its, colour, smell or taste, then the water is considered as najis. 
To conclude, if the volume of water exceeds two qullahs or approximately 200 litres / 52 gallons, then the najis in the water tank doesn’t affect the purity of water on the condition that it doesn’t change the taste, smell or colour of the water. If the najis changes any one of the three characteristics of water, then it is considered as najis. It cannot be used to purify oneself such as for ablution or obligatory bath. Wallahua’lam.
 Qullah is a classical way of measuring water volume. If converted to a modern unit of measurement, two qullahs is approximately 200 kg or 204 kg or 200 litres. Refer Al-Mu’tamad fi Al-Fiqh Al-Syafi’i, 1/41.
 Minhaj Al-Talibin Wa Umdah Al-Muftin, 1/17-19
 Al-Majmu’ Syarh Al-Muhazzab, 1/160